Climate Science Resources

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Weather and climate

An important distinction between the terms “weather” and “climate” is often misunderstood. Take a walk and notice the temperature, humidity, and wind direction. You are observing the weather. Weather happens at a specific location and time, and it changes from hour to hour and from day to day. Climate, however, is the accumulation of weather statistics taken over a long time period, and you need to gather a lot of weather data before you can determine the climate. Take summer in San Francisco as an example. San Francisco , on average, has a maximum daily temperature of 69 degrees F (20.5 degrees C) during July. This climate information, accumulated over the past thirty years of July weather, tells us what the average weather is like for that month. However, on any given day in July, the weather can be quite different, as tourists often discover when the cold fog rolls in under the Golden Gate Bridge . Climate is what you expect the weather to be, but, of course, on any given day the weather can be very different.

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Global Warming vs. Climate Change

Climate change refers to the variations in the Earth’s climate (e.g., temperature or rainfall) over time due to natural or human-related factors. Historically, the Earth’s climate has warmed and cooled many times in response to natural changes such as volcanic eruptions or changes in the intensity of the sun. “Global warming,” however, generally refers to contemporary climate change and the study of the natural and human-related processes responsible for the warming of the last hundred years and projected warming for the next hundred years. Many of today’s scientists are working in the field of global warming; however, knowledge of past climate change is essential to improving our understanding of present and future climate.

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Global Greenhouse Gas Emissions

Each year about 40 billion tons of carbon dioxide equivalent are added to the atmosphere as a result of emissions that are human related.1 Where do these emissions come from? Some greenhouse gas emissions such as carbon dioxide come from burning fossil fuels that are used primarily to generate energy. Fossil-fuel energy comes from petroleum (42 percent), coal (37 percent), and natural gas (21 percent).2 Of the 40 billion tons of carbon dioxide equivalent emitted in the year 2000 (see Figure 1), energy generation from fossil fuels represents about 60 percent of the total emissions.3


globalemissionsFigure 1. Global greenhouse-gas emissions (CO2 e) for different sectors in the year 2000. Thepie chart indicates the percentage of each sector, with the portioning of the energy sector(60 percent) shown on the right. The data were obtained from the Climate Analysis Indicators Too(CAIT) Version 4.0. (Washington, DC: World Resources Institute, 2007) and the analysis includes all available greenhouse gases (CO2, CH4, N2O, PFC, HFC, and SF6). Carbon dioxide equivalent(CO2e) is used to represent the warming potential of all greenhouse gases (i.e., CO2, CH4, N2O, etc.) in a single value.

Energy generation is mainly used for electricity and heat; manufacturing and construction; and transportation. The other major contribution to greenhouse-gas emissions comes from land-use change and agriculture. Permanent removal of forests (or deforestation) leads to CO2 emissions because the carbon stored by trees is released into the atmosphere and is not reabsorbed by the regrowth of new trees. Currently, emissions due to land-use change are largely confined to tropical areas in South America, Africa, and Asia. However, it is the demand by the developed world for wood and other land-based products (for example, meat from cattle and biofuels from sugarcane) that is the driving force in deforestation.
Livestock (especially cows, sheep, and pigs) is the other large contributor to global greenhouse-gas emissions. Cattle emit methane through their normal digestive system and through their manure during decomposition. At present there are an estimated billion-plus head of cattle in the world, and they are responsible for more than 30 percent of the total human-related emissions of methane.4

1. Carbon dioxide equivalent (CO2e) is used to represent the warming potential of all greenhouse gases (i.e., CO2, CH4, N2O, etc.) in a single value.

2. U.S. Department of Energy, Energy Information Administration, Emissions of Greenhouse Gases in the United States 2003 (Dept. of Energy, 2005) ,, accessed Jan. 25, 2008.

3. The data used for this figure come from the Climate Analysis Indicators Tool (CAIT) version 4.0. (Washington, DC: World Resources Institute, 2007). Available at, accessed Jan 25, 2008.

4. H. Steinfeld, et al., Livestock’s Long Shadow: Environmental Issues and Options (Rome: Food and Agricultural Organization of the United Nations, 2006),, accessed Oct. 30, 2007.

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Who is Emitting?

Which countries are responsible for most of the world’s greenhouse-gas emissions? In 2004, the United States had the largest emissions, followed by China and the European Union.1 Recent estimates suggest that China may have overtaken the United States as the largest greenhouse-gas emitter in the world,2which is remarkable because in 1990, China’s total emissions were less than half those of the United States. It should be noted that a large percentage of China’s manufacturing today is for export to the United States and Europe, and without the demand for these goods, China’s emissions (and economy) would be much less. Insight into the relationship between lifestyle and carbon emissions can be found by looking at per capita CO2 emissions, as shown in Figure 1. Of the top emitting countries, the United States has the largest per capita CO2 emissions, at more than twenty tons per year, with Canada and Australia at roughly similar levels. Other developed countries in Europe (such as the United Kingdom, Germany, and France) have about half the total emissions per capita. China’s per capita emissions, although rising, are still only about four tons, about five times less than the average American. Overall, these results imply that North Americans and Australians are particularly heavy greenhouse-gas emitters (compared with Europeans) and suggest that significant reductions could be made without major changes to standard of living.



Figure 1. Per capita annual CO2 emissions in tons per year for a variety of developed and developing countries in 1980 (orange) and 2005 (blue) due to consumption of fossil fuels (petroleum, natural gas, and coal). Data is from the U.S. Dept. of Energy, Energy Information Administration, International Energy Annual, 2005.

1. U.S. Department of Energy, Energy Information Administration, International Energy Annual 2005,, accessed Nov. 1, 2007.

2. Initial estimates reported by the Netherlands Environmental Assessment Agency show that in 2006 China overtook the United States as the largest emitter of greenhouse gases. For details see:, accessed May 8, 2008.

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Weren’t scientists all predicting global cooling in the 1970′s?

No. From the 1940s to the 1970s, the global surface temperature decreased very slightly. This probably occurred because over that time period the cooling effect from human produced aerosols was slightly larger than the warming effect from human produced greenhouse gasses. A minority of scientists predicted that the cooling effect from increasing aerosols would continue to outweigh the warming effect from increasing greenhouse gasses and that the climate would continue to cool. This idea received some public attention when Time magazine published an article titled “Another Ice Age?” in 1974. This article did not represent the views of the majority of the scientific literature at the time (figure 1). For example, a 1975 a paper, published by Wallace Broecker contained the following abstract:

“If man-made dust is unimportant as a major cause of climatic change, then a strong case can be made that the present cooling trend will, within a decade or so, give way to a pronounced warming induced by carbon dioxide. By analogy with similar events in the past, the natural climatic cooling which, since 1940, has more than compensated for the carbon dioxide effect, will soon bottom out. Once this happens, the exponential rise in the atmospheric carbon dioxide content will tend to become a significant factor and by early in the next century will have driven the mean planetary temperature beyond the limits experienced during the last 1000 years.”

This prediction turned out to be remarkably accurate.


figure 1 – Number of scientific papers that predicted cooling, warming, or neutral temperature change by the year of publication.
(figure source: Peterson, 2008)

Broecker, W. (1975) Climatic Change: Are we on the brink of a pronounced global warming?, Science, Vol. 189 no. 4201 pp. 460-463
DOI: 10.1126/science.189.4201.460
Peterson C. T., W. Connolley, J. Flec (2008) The myth of the 1970s global cooling scientific consensus, Bulletin of the American Meteorological Society, 10, 1325-1337, doi:10.1175/2008BAMS2370.

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Why would a few degrees of warming be a big deal?

Because we all experience temperature changes of tens of degrees every day, a few degrees of global warming may not seem like a lot. A few degrees of change in the global surface temperature can, however, indicate a drastic change in the Earth’s climate. This can be exemplified by considering climate changes of the past. The last ice age, for example, was characterized by permanent glaciers over much of North America and Eurasia despite the fact that the Earth was only 5°C-8°C cooler than today. Also, more recent fluctuation in global climate over the last 2000 years have been characterized by temperature swings of only ~0.5°C but have had large enough effects to receive names like the “Medieval Warm Period” and the “Little Ice Age”.

We have experienced only about 0.8°C of warming since the 19th century but the effects are already detectible on many natural systems (IPCC, 2007). Furthermore, this 0.8°C change is small compared to the warming projected for the remainder of the 21st century (figure 1). It is difficult to predict the exact consequences of such warming but studies suggest that more negative outcomes will be seen as more warming occurs (figure 2).


figure 1 – Projected global warming over the next century in the context of climate change over the past 1000 years. Future projections differ depending on estimated changes in human greenhouse gas emissions. (figure source: Chapman and Davis, 2010)


figure 2 – Expected consequences of climate change as a function of future warming (Source)

Chapman D. S., and M. G. Davis (2010) Climate Change, Past, Present, and Future, EOS Vol. 91, No. 37, 325-332
IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 7-22.

Further Reading:
IPCC Climate Change 2007: Working Group II: Impacts, Adaption and Vulnerability Magnitudes of impact

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Past Climate change

The Earth’s climate over the last 4 billion years has changed many times. The swings between warmer and cooler climates are well documented from records such as ice cores or the thickness of tree rings, and indicate that periods of warm and cold climates can last for hundreds or thousands of years.1 These records also indicate that seemingly small changes in global temperature are the difference between an ice age and warmer periods like today. For example, 20,000 years ago, Earth’s average temperature was about 8 degrees F (4.4 degrees C) colder than it is today. That was during our last ice age, when North America was largely covered with ice.2 Step back a further 100,000 years, and the temperature of the Earth was warmer by about 3 degrees F (1.6 degrees C). Most of the Earth’s ice had retreated toward the poles, and sea level was at least twenty feet (6 m) higher than today. 3 So based on historical records, a few degrees difference in the Earth’s global temperature can have a big impact on the planet.


1. The use of tree rings, ice cores, coral reefs, and even historical records such as the time of grape harvest all provide information about the past climate. Information from the NOAA paleoclimatology division provides a good introduction., accessed May 8, 2008.

2. The last ice age was caused by a decline (2 watts/m 2 ) in the surface radiation budget due to orbital variations and feedbacks associated with carbon dioxide and ice sheets. See chapter 6 of the 2007 IPCC report and references therein for details. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to the Fourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007),, accessed May 8, 2008.

3. These data come from the Vostok ice-core temperature reconstruction. You can look at the raw data yourself if you are curious to see how temperature has changed over the last 400,000 years, , or for a good plot of temperature and carbon dioxide over the last 650,000 years, see Figure TS.1. from the 2007 IPCC report. IPCC, Solomon, et al. (2007).

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Didn’t “Climate Gate” show that the science of global warming was based on fabricated data?

“Climate Gate” refers to a situation where the servers of the University of East Anglia were illegally hacked and personal emails from a group of scientists associated with the Climate Research Unit (CRU) were posted online. Thousands of personal emails were published that spanned a time period of over a decade. Out of these thousands of emails, a handful of quotes have been promoted as evidence of a scientific conspiracy.

It should be noted that the science behind global warming has been built up over the last century by scientists at hundreds of different of organizations and therefore would not simply crumble if one group of scientists had been found to be corrupt. Nevertheless this was a serious allegation and it was investigated by at least 5 independent reviews that sought to determine the extent to which scientists had misrepresented findings. These reviews have unanimously found that the scientists under investigation were not fabricating findings and that there is no reason to doubt the science behind climate change.

Below are some selected conclusions from these reports:

“After careful consideration of all the evidence and relevant materials, the inquiry committee finding is that there exists no credible evidence that Dr. Mann had or has ever engaged in, or participated in, directly or indirectly, any actions with an intent to suppress or to falsify data. While a perception has been created in the weeks after the CRU emails were made public that Dr. Mann has engaged in the suppression or falsification of data, there is no credible evidence that he ever did so, and certainly not while at Penn State. In fact to the contrary, in instances that have been focused upon by some as indicating falsification of data, for example in the use of a “trick” to manipulate the data, this is explained as a discussion among Dr. Jones and others including Dr. Mann about how best to put together a graph for a World Meteorological Organization (WMO) report. They were not falsifying data; they were trying to construct an understandable graph for those who were not experts in the field. The so-called “trick” was nothing more than a statistical method used to bring two or more different kinds of data sets together in a legitimate fashion by a technique that has been reviewed by a broad array of peers in the field.”
– RA-10 Inquiry Report: Concerning the Allegations of Research Misconduct Against Dr. Michael E. Mann, Department of Meteorology, College of Earth and Mineral Sciences, The Pennsylvania State University

“We saw no evidence of any deliberate scientific malpractice in any of the work of the Climatic Research Unit and had it been there we believe that it is likely that we would have detected it. Rather we found a small group of dedicated if slightly disorganised researchers who were ill-prepared for being the focus of public attention. As with many small research groups their internal procedures were rather informal.”
-Report of the International Panel set up by the University of East Anglia to examine the research of the Climatic Research Unit.

“Regarding data availability, there is no basis for the allegations that CRU prevented access to raw data. It was impossible for them to have done so.”
“Regarding data adjustments, there is no basis for the allegation that CRU made adjustments to the data which had any significant effect upon global averages and through this fabricated evidence for recent warming.”
“We do not find that the data described in AR4 and shown in Figure 6.10 is misleading, and in particular we do not find that the question marks placed over the CRU scientists’ input cast doubt on the conclusions.”
“In summary, we have not found any direct evidence to support the allegation that members of CRU misused their position on IPPC to seek to prevent the publication of opposing ideas.”
-The Independent Climate Change E-mails Review

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How can there be global warming when last year was so cold?

Play Video (9:38)

This episode defines climate and weather, and provides a brief introduction to climate change.

Topics covered in this video:

-10 year moving average vs. 12 month moving average
-Long term change vs. Year to year variability
-Local temperature and cold/warm air masses
-Global average change vs. Local average change
-Temperature anomalies for 2010
-Time series plots for Washington D.C. and other major cities from 1750-2010
-Comparison of global time series plots

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Why do we think climate change is being caused by humans?

Play Video (11:34)

This episode reviews the lines of evidence that suggest human activities are the cause of climate change.

Topics covered in this video:

Fundamental physics of Greenhouse Effect
-Gas absorption of radiation
-Documented human caused increase in atmospheric Greenhouse gases
-Graphical relationship of temperature increase and Greenhouse gas increase
Signatures of an enhanced Greenhouse Effect
-Measuring Greenhouse gases and observed changes in long wave radiation
-Ocean circulation changes and graphical representation of temperature change on land and oceans.
Natural phenomena don’t explain observed warning
-Milankovitch cycles, formation of continents, biogeochemical processes
-Volcanic eruption, solar radiation, and Greenhouse gas increase plots

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What is the greenhouse effect?

Play Video (10:14)

This episode reviews the greenhouse effect and how it relates to human activities and global warming.
Topics covered in this video:
-Natural Greenhouse Effect without human interaction
-Solar radiation and albedo relationship
-Infrared radiation and Greenhouse gas relationship
-Incoming radiation vs. Outgoing radiation
-Wavelengths: short vs. long
-Gas absorption of different wavelengths of radiation
-Greenhouse gas properties (trace gases)
-Transparency vs. Opaqueness
-Major GHG’s (H2O, CO2, CH4, N2, O3)
-How a Greenhouse actually works

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Is the Earth getting warmer?

Play Video (10:47)

This episode looks at the evidence that suggests the Earth is getting warmer.

Topics covered in this video:

-Meteorological Stations
-Records from 1880-2010
-1.2 Degree rise
-Weather Balloons
-Records form 1950-2010
-Records from 1979-2011
-Frozen water (Cryosphere) measurements
-Photographical evidence (4:12)
-Length of glaciers (4:40)
-Arctic sea ice measurements (5:30)
-Mass of ice sheets and gravitational pulls (7:15)
-Snow coverage and frozen ground

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Hasn’t climate changed before?

Play Video (7:08)

This episode explains when and how the climate has changed before and how it differs from current climate change.

Topics covered in this video:

-Introduction (Yes!)
-Measurements of CO2 content from 400 million years: relationship to glaciers and temperature
-More recent temperature oscillations (500,000 year timeline) due to Milankovitch cycles
-CO2 effect on intensity of temperature change
-1200 year timeline
-Tree ring width, chemical signatures in ice cores, and meteorological records
-Logic of past climate changes and current climate change causes

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How difficult will it be to limit future climate change?

Play Video (9:09)

This episode talks about our ability to limit greenhouse gas emissions to avoid potential impacts.

Topics covered in this video:

-Sources of Greenhouse gases
-Emissions vs. Concentrations
-Scenarios of CO2 emissions and temperature change
-Reduction of 80% and leveling of concentration
-Current CO2 emission standings

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Are Humans Increasing Greenhouse Gas Concentrations?

Play Video (11:38)

This episode examines the human contribution to greenhouse gas concentrations in the atmosphere.

Topics covered in this video:

-Small percentage of CO2 can cause spike in CO2 concentration
-Carbon cycle
-Description of flux between atmosphere, biosphere, and hydrosphere
-Industrial revolution carbon flux and imbalance
-Bathtub analogy
-Accumulations of small percentages
-Graph of CO2 concentrations
-Laws on CO2 control
-Plant and animal interaction (internal)
-External Sources

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What are the potential impacts of climate change?

Play Video (11:04)

This episode reviews scientific reports that summarize potential climate change impacts.

Topics covered in this video:

-Temperature anomaly over past couple hundred years and current rate of warming
-Temperature changes in New York and Washington D.C.
-Location and local climate movement
-Mean increase diagram
-Extremes (More record high’s)
-Ratio of record high’s and low’s
-Precipitation change
-Future projections 2090-2099
-Heavy rain, droughts, and floods
-Sea level rise
-IPCC chart on Global Warming impacts

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What would cause Earth’s temperature to change?

Play Video (6:43)

This episode explores the factors that would cause the Earth’s climate to change.

Topics covered in this video:

-Fundamentals of Physics: Temperature
-Energy Budget
-Family Budget Analogy (Income Earnings and Spendings)
-Earth’s system relationship to family budget analogy
-Incoming solar radiation vs. infrared radiation

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Why don’t CO2 and temperature correlate perfectly?

Play Video (7:34)

This episode discusses the factors that explain why carbon dioxide and temperature don’t correlate perfectly.

Topics covered in this video:

-Global Temperature and CO2 graphical representation
-Decade to Decade time scale variability
-Graphs of isolated scenarios for temperature change (Anthropogenic greenhouse gases, Anthropogenic Aerosols, Volcanic Aerosols, and Solar radiation)
-Graphs of all influences combined
-Internal Variability (changes in ocean heat absorption)
-Fictional Example of isolated Internal Variability and example with combined Greenhouse Gases

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How can we predict the weather 100 years from now?

Play Video (6:15)

This episode talks about the methodology for predicting the average weather in the future.

Topics covered in this video:

-Weather forecasts are uncertain after 5-8 days
-We can’t!
-Wave analogy
-Individual waves are random, but tides are predictable
-Dice analogy
-Converges to number 7 after hundreds of rolls
-Graph of San Jose 2013 temperature average and long term trends

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Introduction to Earth’s Climate System

Play Video (14:33)

This episode defines climate and weather, and provides a brief introduction to climate change.

Topics covered in this video:

0:00 – 2:55
– Difference between climate and weather
– Why is climate so difficult to understand?
2:56 – 6:07
– How climate changes naturally (e.g. ice ages)
– How humans have changed land use (e.g. pastures and croplands)
6:08 – 8:52
– Average world temperarture and sea level
– Arctic sea ice and ice proxies
8:53 – 12:00
– CO2 concentrations
12:01 – 12:33 (end)
– Greenhouse gases N2O, CH4
– Overview of how all 3 gases (CO2, N2O, CH4) show the same exponential increase with human population
– Ice cores

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Complex Systems and Feedbacks

Play Video (19:16)

This episode investigates systems and feedbacks to understand how climate operates.

Topics covered in this video:

0:00 – 3:28
– What is a complex system?
– Linear vs non-linear
3:29 – 8:53
– Equilibrium and stability
8:54 – 12:17
– Feedbacks (positive and negative)
– Examples of feedbacks
12:18 – 19:10 (end)
– Climate forcings and feedback loops
– Feedback examples: ice-albedo, cloud coverage, bobcats and rabbits

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Global Climate Change: Paleoclimate

Play Video (18:28)

This episode continues our investigation of current climate change by exploring the climate of the past and the techniques used to do so in present day.

Topics covered in this video:

0:00 – 1:24
– Defining paleoclimate
– Why study paleoclimate?
– Proxies and why they’re used
– What types of proxies are there?
1:25 – 3:02
– Tree rings as proxies
– How they are used
3:03 – 7:59
– Ice cores as proxies
– Isotopes (hydrogen and oxygen), bubbles
– Antarctica and Vostok
– How ice cores are used
– Figure of ice core data
8:00 – 9:36
– Sedimentary particles (fossils and pollen) as proxies
9:37 – 12:58
– Diagnostic rock types
– Coal deposits, salt or gypsum deposits, glaciers
– Evidence of past glaciers
– Moraine, smoothing striations, erratics
12:59 – 18:28 (end)
– Maps showing the extent of ice in North America
– Relation to pine trees receding north
– Pollen trends
– Deglaciation of North America
– Lakes near the edge of glaciers
– Breakout of fresh water to North Atlantic and its effect on the Great Ocean Conveyor Belt (Younger-Dryas cooling)
– Plot of Greenland temperature during the Younger-Dryas cooling

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Global Climate Change: Review and Human Impact

Play Video (8:52)

This episode reviews how the climate has been undergoing an accelerated change in the last couple hundred years, and it pointsout the evidence that suggests certain human activities are contributing to it.

Topics covered in this video:

0:00 – 3:01
– Greenhouse gas values of CO2, N2O, CH4
– world population growth
– exponential functions
3:02 – 6:44
– acceleration of human activity
– fertilizer consumption
– water use
– foreign investment
– energy use by humans (by era)
6:45 – 8:52
– How some human activities influence climate
– burning fossil fuels
– fertilizers and agriculture
– livestock
– deforestation
– modern technology

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Climate Models and Social Scenarios

Play Video (11:58)

This episode explains how scientists use analytical models to evaluate climate, involving both natural and human forcings.

Topics covered in this video:

0:00 – 1:54
– Earth’s climate system
– A brief definition of global climate models
– General Circulation Models
– Diagram of a complete 6-Box climate model (very complicated)
1:55 – 4:33
– Grid Spacing
– How climate models handle small scale features
– Parameterizations
4:34 – 8:49
– Simulation tests
– Simulating recent climate
– Can simulate natural forcings only or natural and anthropogenic
8:50 – 10:44
– Climate forcing diagram
10:45 – 11:58 (end)
– Tipping points
– Feedbacks cause a more complex climate

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Projections based on Models/Projections

Play Video (12:15)

This episode investigates social scenarios of the future and how they will influence climate.

Topics covered in this video:

0:00 – 0:25: Introduction
0:26 – 2:13
– How are ideas communicated?
– How are climate change ideas communicated?
– Intergovernmental Panel on Climate Change (IPCC)
2:14 0 6:05
– What choices will humans make in the future about land use, resource exploitation and consumption, etc?
– IPCC Scenarios
– Main families: A1, A2, B1, and B2, with an explanation of each one
6:05 – 12:15 (end)
– Examples of IPCC scenarios
– Using scenarios in climate projections
– Carbon dioxide from 2000 – 2100
– Projections for the 21st century under all scenarios
– Projected temperature increase with the A2 scenario

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Greenhouse Effect

To understand the science of global warming, we can start by looking at a greenhouse. Essentially a small house made of glass, a greenhouse allows the sun’s energy to pass through easily while inhibiting the heat from leaving. The Earth’s atmosphere works in a similar way. A majority of the sun’s radiation passes through the cloudless atmosphere and acts to warm the Earth’s surfaces and the oceans. In turn, the land and ocean give off energy that is headed out to space. Most of this energy does not escape to space, rather it is absorbed by naturally occurring greenhouse gases such as water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). After they absorb the Earth’s energy, they, in turn, re-emit some of that energy back toward the Earth’s surface and provide additional warming of the Earth. This is what we call the greenhouse effect because, although the sun’s energy is able to pass through the greenhouse gases, these same greenhouse gases trap the Earth’s outgoing energy.


It turns out that greenhouse gases play a very important role in our climate system. Remove all the greenhouse gases in the atmosphere and the Earth’s average temperature would be a very chilly 0 degrees F (-18 degrees C), and most of the planet would be frozen. However, in a manner similar to putting blankets on to trap your body heat when you go to bed, the layer of greenhouse gases acts as a blanket to keep the Earth a comfortable temperature of 59 degrees F (15 degrees C). And just as you can expect to sleep warmer if you add extra blankets to your bed, you can expect the Earth to get warmer if you add extra greenhouse gases to the atmosphere. So where are these greenhouse gases and how many of them are around? Well, take a deep breath. You have just inhaled an assortment of molecules that primarily include nitrogen and oxygen, and also small amounts of greenhouse gases such as water vapor and carbon dioxide. It’s actually these molecules in relatively small concentrations, which exist everywhere in the atmosphere, that turn out to be important for shaping our climate. (You can now exhale!)

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How do we know that the world is warming?

Scientists use data from meteorological stations, ships, buoys and satellites to construct instrumental records of temperature as far back as the late 19th century. These temperature reconstructions show a significant warming of the globe over this time period (figure 1).


figure 1 – National Aeronautics and Space Administration (NASA) global surface temperature over the period 1880-2010 expressed relative to the average temperature from 1951-1980 (Source)

In addition to directly measuring temperature changes, scientists can infer temperature changes by making observations of the natural world. Many natural systems are changing in a manner that is consistent with warming. Such systems include, but are not limited to, alpine glaciers (figure 2), ice sheets (figure 3), Arctic sea ice (figure 4) and sea level (figure 5).


figure 2 – World Glacier Monitoring Service (WGMS) global glacier mass balance over the period 1945-2005

IceMassGreenland IceMassAntarctica

figure 3 – Satellite derived Ice mass in Greenland and Antarctica over the period 2002-2009 (Source: Velicogna, 2009).


figure 4 – University of Washington Polar Science Center arctic sea ice volume over the period 1979-2011. (Source)


figure 5 – Global sea level over the period 1870-2008 (Source)


Further Reading:
NASA Global Climate Change Key Indicators

Velicogna, I. (2009), Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophys. Res. Lett., 36, L19503,doi:10.1029/2009GL040222.

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Hasn’t climate changed before?

Yes. Climate change has occurred many times in the Earth’s 4.5 billion year history due to a variety of causes. Scientists use these past changes in climate to better understand the changes we are currently experiencing.

Many of Earth’s previous climate changes were due to factors that changed slowly over long periods of time (tens of thousands to hundreds of millions of years). These factors include changes in the position and drift rate of continents as well as changes in Earth’s orbit around the Sun. Because these factors only affect climate on very long time scales, they cannot be responsible for the warming that we have seen over the past century.

Natural factors that affect Earth’s climate on centennial time scales include changes in the brightness of the sun as well as changes in the frequency of large volcanic eruptions. These two factors likely dominated natural climate variability over the past 1000 years prior to the industrial revolution (Crowley, 2000). These fluctuations have not, however, been of the same magnitude as the changes that are currently being experienced (figure 1).


figure 1 – Reconstruction of Northern Hemisphere temperatures over the last 1300 years (Source)

Further Reading:
EPA Past Climate Change

Crowley, T.J. (2000) Causes of climate change over the past 1000 years. Science, 289 (5477), 270–277.

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Carbon Dioxide

An interesting illustration of human impact on the Earth’s atmosphere comes from Hawaii . Since 1958, daily measurements of carbon dioxide have been taken from the Hawaiian mountain of Mauna Loa , at 13,680 feet (4,170 m) above sea level.

In Figure 1, the solid black line is the five-year average measurement mean; the red line represents the actual monthly measurements. The average CO2 concentration has steadily increased over the last fifty years. Because carbon dioxide is relatively well mixed in the atmosphere, the measurements at Mauna Loa are a good estimate of the global average. Using ice-core measurements from Antarctica , scientists believe that today’s concentration of carbon dioxide is higher than at any time over the last 650,000 years.1


Figure 1. Monthly values of atmospheric carbon dioxide concentrations at Mauna Loa Observatory , Hawaii , measured in parts per million (ppm) indicating the number of molecules of carbon dioxide per million air molecules. The black curve represents the smoothed data.2

In addition to the steady rise in carbon dioxide, the annual rise and fall of carbon dioxide in the atmosphere is also striking. Every year CO2 concentrations go through a cycle in which they slightly decrease between April and July and slightly increase in September and October. This regular pattern reveals the connection between the atmosphere and all living plant matter on Earth. Here is how it works. In spring and early summer, new growth on trees, plants, and grasses returns with the warming weather. Because plants take in carbon dioxide as they grow, CO2 levels start to decrease, and by the summertime – the peak growing season – CO2 levels have noticeably dropped. In the cooler weather of fall, plants start to lose their leaves which break down and return carbon to the atmosphere, and CO2 levels then start to increase. Some people have suggested that the annual cycle in carbon dioxide is an example of the Earth breathing. During the spring and summer, the Earth’s plant life is growing and thus taking in carbon dioxide, whereas in fall and winter, plants have died back and carbon dioxide is returned to the atmosphere.

1. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to theFourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007).

2. The data on Mauna Loa constitute the longest record of direct measurements of carbon dioxide in the atmosphere. Data from the Scripps Institute of Oceanography are in blue and from NOAA in red. An updated (every month) figure from NOAA can be seen at:

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A Warmer World

As the Earth warms, we can expect to see a variety of changes. Direct changes that have been documented during the twentieth century include higher maximum and minimum temperatures; changes in the amount, location, and intensity of precipitation; and a steady increase in sea level. Other manifestations of a warmer world include increases in the severity of drought and in flooding associated with extreme precipitation events.1 For example, Australia is currently in the midst of its worst drought in at least a century, with scientists predicting that parts of inhabited Australia will essentially remain drier during the coming century.2 While many cities have been forced to enact severe water restrictions (e.g., Brisbane imposed level-five restrictions in 2007)3, a number of cities have declared that some level of water restrictions will become permanent.

The increased occurrence of drought and floods are in part related to shifts in storm tracks and thus shifts in rainfall patterns. Since modern civilizations have developed around access to freshwater, if there are changes to the locations of freshwater due to lack of rain, inadequate snow/ice melting, or coastal intrusions of saltwater into freshwater lakes, then the viability of some societies could be significantly compromised.

Other signals of a warmer world include the increase in intensity of tropical cyclones. Although the connection between hurricane intensity and human induced warming is still an area of active research,4 rising sea levels and, more importantly, increases in coastal populations will make the types of damage seen in New Orleans in 2005 more likely in the future.

Less controversial but just as devastating are the increased occurrences of extreme heat events and heat waves.5 The most prominent recent example is the European heat wave of 2003, in which more than 30,000 people died as temperatures soared for weeks.6 Although no single weather event can be attributed to climate change, some research suggests that human-influenced global warming has doubled the risk of heat waves of this magnitude.7 It is also clear that if the warming continues, events such as these are expected to occur more frequently. The impacts of a warmer world affect other biological life on Earth as well. The earlier arrival of spring and the overall warmer minimum temperatures have a particularly important influence on bird and insect migration. For example, disease-carrying mosquitoes are expected to spread to higher altitudes and other areas previously too cold for their survival. Many examples of more northerly migrations are already occurring, ranging from the Pacific starfish slowly migrating north along the Californian coast 8 to the recent arrival of the famously colored puffin birds in northern Alaska.9 These migrations change the natural balance between food and predators and may subject some species to further danger.

1. A good summary of the observed twentieth-century changes and the predictions for the twenty-first century are given in the IPCC Fourth Assessment Report: Summary for Policymakers section of the report. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to theFourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007),

2. B. L. Preston and R. N. Jones, “Climate Change Impacts on Australia and the Benefits of Early Action to Reduce Global Greenhouse Gas Emissions” (2006),, accessed Oct. 31, 2007.

3. Level 5 restrictions include no watering of lawns, limited watering of established gardens on certain days and during certain times, and no filling of pools unless certain other water saving measures have been taken. The Web site of the Brisbane City Council contains more information about Brisbane’s water situation, accessed May 8, 2008.

4. At present, there is debate within the scientific community about the connections between tropical cyclones and global warming. While some studies show an increase in the intensity of tropical cyclones over the past few decades, others attribute this to observational techniques and instrumentation. Two good sites devoted to the science are

5. IPCC, Solomon, et al. (2007).

6. T. Kosatsky, “The 2003 European Heat Waves,”Euro Surveillance 2005, 10 (2005), 148-49.

7. P. A. Stott, et al., “Human Contribution to the European Heatwave of 2003,”Nature, 432 (Dec. 2, 2004), 610-614.

8. B. Wuethrich, “How Climate Change Alters Rhythms of the Wild”Science, 287 (2000), 793-95.

9. C. Gjerdrum, et al., “Tufted Puffin Reproduction Reveals Ocean Climate Variability, “Proceedings of the National Academy of Sciences, 100 (2003), 9377-82.

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How can we project climate change out toward the end of the century when we can’t even predict the weather 2 weeks from now?

Local weather is the result of somewhat random fluctuations in the flow of the atmosphere around the planet. This makes the weather at any given location difficult to predict past about a week in time. Climate, or average weather, on the other hand, is largely determined by external forcings that are more predictable.

This idea is exemplified each year in the seasonal cycle. Summer is warmer than winter because days are longer and the sun is higher in the sky. Even though any given location will experience random weather fluctuations, we can count on summer being warmer than winter because the changes in solar radiation dominate any random weather changes over periods longer than a couple months in length. Similarly, changes in global climate can be projected into the future because on the decade to decade scale, changes in greenhouse gasses dominate random fluctuation in weather.

Further Reading:
IPCC FAQ What is the relationship between climate change and weather?

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Carbon Cycle

Carbon exists in all living things and has been called the building block of life. Carbon also exists in nonliving things, such as carbon dioxide (an invisible gas) and rocks such as limestone. Our understanding of how carbon moves between the atmosphere, ocean, and land is central to quantifying how increases in greenhouse gases and changes in land surface (i.e., deforestation and soil erosion) will affect climate in the future. The carbon cycle is a complex series of processes that describes how carbon is exchanged between the carboncycleatmosphere, ocean, and land (i.e., plants, soil, and the Earth’s crust). One component of the carbon cycle is the exchange of carbon between the atmosphere and plants (i.e., carbon dioxide absorbed by plants via photosynthesis), while volcanic eruptions that inject carbon dioxide into the atmosphere represent an exchange of carbon between the Earth’s crust and the atmosphere. Although carbon is constantly moving between different parts of the Earth, the total amount of carbon on the planet is constant. In this way, if the amount of carbon in the ocean were to go down, then the amount of carbon in the atmosphere or land would have to go up. This property of carbon conservation allows us in theory to keep track of all carbon movements throughout the Earth.


Today, the burning of coal and petroleum for energy is just an acceleration of one component of the carbon cycle. For example, the carbon in coal would normally remain in the ground for thousands of years until erosion or plate tectonics eventually released this carbon back into the atmosphere. So when humans burn fossil fuels they are really just moving carbon from the land into the atmosphere.1 Well-defined estimates exist for how much carbon goes into the atmosphere each year due to the burning of fossil fuels and changes in the land surface. There are also various organizations that are working to take carbon out of the atmosphere and put it back into the land. For example, the planting of a tree over a period of time will store atmospheric carbon in the trunk, branches, and roots of the tree. However, true to the carbon cycle, if that tree were to burn in a fire, the carbon would be liberated back into the atmosphere. So careful monitoring of the carbon cycle is necessary to understand how human factors affect atmospheric carbon levels and climate.

1. Fossil fuels are a nonrenewable resource, and limited reserves exist on our planet. Estimates vary as to how many years each energy source will be readily available at an acceptable economic cost, and range from ten to fifty years for oil, thirty-five to eighty years for natural gas, and one hundred to two hundred years for coal.

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How can there be global warming when last week/month was so cold?

The local temperature at any given location is mostly determined by changes in the flow of heat around the Earth’s surface. This idea is illustrated in weather maps that represent the movement of cold and warm air with fronts (figure 1). If cooler air moves over a particular location this will generally be cancelled out by warmer air moving over some other location. In fact, the Earth’s average surface temperature only varies by about 0.1-0.2°C from year to year. This means that local weather is a poor indicator of the global temperature and therefore it is quite possible for a given location to experience prolonged cool periods in spite of global warming.


figure 1 – Typical North American weather map

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Twentieth Century Warming

Daily observations of temperature have been gathered from meteorological stations round the globe for many decades. Figure 1 shows global temperatures from 1850 to the present. The most obvious feature is the gradual warming of the planet, although you can also notice decades when the global temperature remained constant or even cooled. These variations reflect the competition between warming and cooling factors, with both natural and human origin. However, the most prominent signal of change has been the steady warming of the Earth since 1970: the warmest years on record (1998 and 2005), the warmest decade on record (the 1990s), and by 2007, eleven out of twelve of the warmest years have been in the past twelve years.1 Clearly, over the last century, our planet has been warming. Other signals of a warming planet or “fingerprints” of climate change have also been identified.2 Fingerprints include the melting of glaciers, the rise in sea level, and the changes in distribution of plant and animal species. Today most glaciers in the world are in retreat, some of them very rapidly.3 Sea-level rise has also been observed around the world.4 Plants and animals are also sensitive to temperature, and migrations in numerous insect and marine animal populations toward higher altitudes or higher altitudes (where it’s cooler) have been observed.5 Taken together, these collections of fingerprints present further evidence that the planet is warming.


Figure 1. Observed changes in (a) global average surface temperature, (b) global average sea level where tide-gauge data is in blue and satellite data is in red, and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas represent the uncertainty.






1. P. Jones, “Global Temperature Record,” Climate Research Unit Information Sheet ( Norwich : University of East Anglia, 2007),
info/warming/, accessed Oct. 30, 2007.

2. A couple of papers on fingerprints can be found at
nature/links/030102/030102-3.html and a climate fingerprints hot map at, accessedMay 1, 2008.

3. The Grosser Aletsch Glacier in Switzerland , the longest glacier in the Alps, has retreated 8,500 feet (2,600 m) since 1980, and the Rongbut Glacier, which drains the north side of Mount Everest into Tibet , has been retreating 65 feet (20 m) per year over the last few decades. In 2006, the Swiss Glacier survey of 85 glaciers found 84 retreating and 1 advancing. Similarly, of the glaciers in the Italian Alps, only about a third were in retreat in 1980, while by 1999, 89 percent of these glaciers were retreating. In 2005, the Italian Glacier Commission found that 123 glaciers were retreating, 1 advancing and 6 stationary,
messntz/glacierlist.html, accessed May 1, 2008.

4. On the Pacific island of Tonga , the sea level appears to have risen about 0.3 inches (8 mm) a year over the last fifteen years. Although this may not seem like much, during times of storms, low-lying islands are expected to see an increased threat of flooding within the next few decades. Sea level rise is not constant, and depends on local variations in ocean temperature and winds. Further information on Pacific Islands and sea level can be found at
pacificsealevel/index.shtml, accessed May 1, 2008.

5. C. Parmesan and G. Yohe, “A Globally Coherent Fingerprint of Climate Impacts across Natural Systems,” Nature, 21 (2003), 37–42. 18. IPCC, Climate Change 2007: The Physical Science Basis: Contribution of Working Group 1 to the Fourth Assessment Report on the Intergovernmental Panel on Climate Change, S. Solomon, et al., eds. (Cambridge: Cambridge University Press, 2007), Fig SPM.3,, accessed May 8, 2008.

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Don’t humans produce just a small percentage of the total carbon that is emitted into the atmosphere?

Carbon is constantly moving between the atmosphere, ocean, land and biosphere (figure 1). Before humans started burning fossil fuels, all this movement of carbon was balanced so that, over the previous few thousand years, there was no net change in the amount of CO2 in the atmosphere. Ever since the industrial revolution, however, the burning of fossil fuels has introduced a new source of carbon that did not previously exits. This new source is only a small part of the total carbon cycle but it has caused the concentration of CO2 in the atmosphere to increase by nearly 40% (figure 2). Therefore, it is true that humans produce only a small percentage of the total carbon that enters the atmosphere in any given year, however, humans are responsible for almost 100% of the increase in atmospheric CO2 that has occurred since the industrial revolution.


figure 1 – The global carbon cycle in petagrams of carbon (Source)


figure 2 – Concentrations of major greenhouse gasses over the last 2000 years (Source)

Further Reading:
IPCC Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities?

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Climate Models

Probably the most important tools in use today for understanding climate change and predicting the future are global-climate models. These highly sophisticated models are mathematical descriptions of the atmosphere and ocean. Similar to weather-forecast models that predict the weather within the next few days, global-climate models can develop projections for how the climate will change over the next few decades. And just as we know that weather-prediction models sometimes yield the wrong answer, climate models also have their limitations. So, how do we know that today’s climate models are good enough to make accurate predictions of the future? The test for climate models is to see if they can reproduce the climate of the twentieth century, for which there are lots of observations for comparison. We start the model in the year 1900 and simulate the climate for one hundred years. We can also run experiments where we pose different questions, such as, “What would the climate of the twentieth century be like if humans were not around?” The results of such a series of experiments are shown in Figure 1, where the thick black line indicates the observed global temperature, and the colors indicate the results of various climate-model predictions during the twentieth century. The blue shading indicates the global temperature in simulations that include only natural changes to our climate system, such as changes in solar radiation and the major volcanic eruptions of the twentieth century. The red shading indicates simulations that include both natural changes and human influences (that is, emissions of greenhouse gases and aerosols). The model simulations that include both natural and human processes actually match the observations fairly well, whereas the models that neglect human-produced gases do not show the warming trend observed since 1970.


Figure 1. Comparison of global surface temperature with results simulated by climate models using natural and human forcings.1






These experiments therefore demonstrate two important points. First, it is very likely that human-produced greenhouse gases are responsible for a large fraction of recent warming, especially over the last fifty years.2 And second, they provide us with confidence that model predictions of the next one hundred years will have some legitimacy.

1. Decadal averages of observations are shown for the period 1906 to 2005 (black line) and blue-shaded bands show the range for simulations from five climate models using only the natural forcings due to solar activity and volcanoes. The red-shaded bands show the range from fourteen climate models using both natural and human forcings. Forcings refer to processes that will act to either warm or cool the planet such as changes in the sun’s radiation or changes in the concentration of a gas like carbon dioxide. Forcings can either be natural (e.g., volcanoes that act to cool the planet, or increases in the sun’s radiation that would warm the planet) or human-produced (e.g., increases in carbon dioxide that will warm the planet, or increases in aerosols that will cool the planet). IPCC, Solomon, et al. (2007), adapted from Figure SPM.4.

2. These results were produced using fourteen different climate models from international research groups in support of the 2007 IPCC report (Solomon, et al). Further and related details can be found in the FAQ chapter under FAQ 8.1: How reliable are the models used to make projections of future climate change?” and FAQ 9.2: “Can the warming of the 20th century be explained by natural variability?”