Understanding Climate Change: A Primer

This primer provides a broad overview of the main issues of Climate Change. Additional resources and more specific information are available throughout the Our Work section of this website.

Understanding a phenomenon known as the greenhouse effect is a first key step in understanding how climate change is affecting our planet.

Solar radiation interacts with the surface of the earth. This interaction takes several forms: some portion of incoming solar energy is reflected back into space by the earth’s atmosphere; another portion is dispersed and scattered by the molecules in the atmosphere; and a large portion penetrates through the earth’s atmosphere to reach the planet’s surface. The radiation reaching the earth’s surface is largely absorbed, resulting in surface warming (Figure 1).

Much of this absorbed energy is eventually re-radiated in longer infrared wavelengths. As it leaves the earth, it once again interacts with the atmosphere. Some of this re-radiated energy escapes to space, but much of this re-radiated energy is reflected back to the earth’s surface by molecules in the earth’s atmosphere. This phenomenon is similar to the warming that occurs in an automobile parked outside on a sunny day (Figure 2).

Figure 1.
This image illustrates the greenhouse effect.


Figure 2.
A parked car in the sun creates a localized greenhouse effect.

The molecules responsible for trapping re-radiated energy in the earth’s atmosphere are called greenhouse gases because they act like the glass in a greenhouse. The most important greenhouse gases include water (H2O), nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). Without these gases, most life on earth would not be possible, as the surface temperature of the earth would likely be about 60°F colder.

Greenhouse gases act like an insulator or blanket above the earth, keeping the heat in. Increasing the concentration of these gases in the atmosphere increases the thickness of this insulator, therefore increasing the atmosphere’s ability to block the escape of infrared radiation. Too great a concentration of greenhouse gases can have dramatic effects on climate and significant repercussions for Earth. Too low a concentration can have dramatic effects as well. Climates suitable for human existence are limited above some minimum threshold level of greenhouse gas concentration. In other words, those climates are possible only within a finite window – a limited range of greenhouse gas concentrations make life as we know it possible.

Increasing Temperatures & Greenhouse Gases

Through the study of ice cores from Antarctica, atmospheric concentrations of the dominant greenhouse gas, carbon dioxide (CO2), can be determined over hundreds of thousands of years. Figure 1 illustrates variations in both atmospheric CO2 concentrations and temperature over the past 400,000 years. A comparison of the two trends indicates a very tight connection between their performances, with fluctuations in one curve almost exactly mirrored in the other. Periods of higher CO2 concentrations are warmer (interglacial); periods with lower concentrations are colder (glacial). In the 1800s – as the Industrial Revolution started – atmospheric CO2 concentrations began an unprecedented upward climb, rising rapidly from 280 ppm (parts per million) in the early 1800s to a current level of 397 ppm, as of 2009. The current concentration is 38 percent higher than it was at the start of the Industrial Revolution.


Figure 1. CO2 and Temperature. Barnola et al, 2003. Data from Oak Ridge National Laboratory.

The Intergovernmental Panel on Climate Change (IPCC)

Noting these trends, and recognizing the potential for dramatic changes in the climate due to continued unchecked accumulation of greenhouse gases in the atmosphere, the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) established the Intergovernmental Panel on Climate Change (IPCC) in 1988. The purpose of the IPCC is to review existing and developing peer-reviewed scientific literature to form an objective evaluation about the risk of human-induced climate change.

After years of investigation, and in consultation with thousands of scientists, the IPCC was able to write, in its Fourth Assessment Report in 2007, that “warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.” Their report noted that the dramatic increase in carbon dioxide concentration in the atmosphere over the past 150 years is largely due to anthropogenic (human-caused) effects and concluded that “most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.”

The IPCC’s models predicted a rise of 1 to 5 degrees Celsius (2.0 to 11.5 degrees Fahrenheit) in the global mean surface temperature during the next century, with sea-levels expected to rise by between 7 and 23 inches (excluding possible future rapid changes in dynamical ice flow) by 2100. (IPCC 2007). The IPCC continues to play a central role in reviewing and assessing the most recent information on climate change.

Improved Models, Growing Confidence

The Fourth Assessment Report of the IPCC, released in 2007, added weight to the linkage between rising temperatures and continued greenhouse accumulations.


Figure 2. a) Comparison between global mean surface temperate anomalies from observations (black) and model simulations forced with anthropogenic and natural forcings, and b) forced with natural forcings only. The labeled verticals lines in Figure 2 represent major volcanic events, which generally have a temporary cooling effect on the global climate. Source: IPCC, Fourth Assessment Report, 2007.

For example, recorded global temperature change can be compared with computer models that predict temperature change under different “forcing” – or external influences on the underlying radiative budget of the planet – scenarios. Forcings may include greenhouse gases, aerosols, solar radiation, and other agents). Figure 2 compares observed temperature differences from a historic mean (black lines) with the results of computer models that attempt to predict temperature based on the interactions of other environmental influences (red and blue lines).

Chart B in the figure illustrates that models using only natural influences fail to match the observed record of temperature anomalies since 1900. But the combination of natural and anthropogenic models, as illustrated in Chart A, produces a close match to the observed data. Climate models thus help reveal a clear “thumbprint” of human impacts on climate change.

Based on results such as these, the IPCC’s 2007 report stated emphatically that “for the next two decades, a warming of about 0.2°C (0.35°F) per decade is projected for a range of emission scenarios. Even if the concentration of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1oC (0.2°F) per decade would be expected.”

Accumulating Evidence

Other evidence of climate change continues to accumulate. Consistent with predictions of the IPCC since 1990, global average temperatures have indeed been rising, while the rate of atmospheric CO2 has also been increasing (Figure 3). The rate of growth in CO2 concentrations in the first eight years of the 21st century was more than twice the rate observed in the 1960s (Le Quéré et al., 2009).


Figure 3. Global Temperature & CO2 Concentration Since 1880. Data from NOAA’s National Climate Data Center (NCDC) & Oak Ridge National Laboratory.

Fourteen of the fifteen warmest years on record since 1850 have occurred in the last fourteen years. In other words, only one year in period 1995 – 2009 (1996) is not one of the fifteen hottest years on record. The warmest year was 1998, followed by 2005, 2003, 2002, 2004, 2009, 2006, 2001, 2007, and 1997, as cited by the Climatic Research Unit, University of East Anglia. The 1990s were the warmest complete decade since 1850, and was, on average, 0.43 degrees Celsius warmer than the period 1961-1990.

Other events illustrate the climatic changes that are likely to become more prevalent under a changing global climate regime:

Glaciers are present on every continent other than Australia and function as reasonably well-distributed indicators of changing global temperatures. Worldwide, glaciers and icefields have been shrinking and receding for at least the last century. The collapse of the 1250 square mile Antarctic Larsen B ice shelf in 2002 was just one of the more spectacular instances of a phenomenon that is likely to become more frequent in a warmer world.

While the Antarctic may actually see some areas of growth in its ice sheet due to increased precipitation under a changing climate regime, the northern Arctic region appears to be even more vulnerable. In a 2004 report by the Arctic Monitoring and Assessment Programme (AMAP), (Impacts of a Warming Arctic: Arctic Climate Impact Assessment), the list of potential changes in the Arctic due to warming includes such phenomena as decreases in sea ice, increasing precipitation and river discharge, thawing of glaciers and permafrost, and changes in plant and animal abundances and distributions.

While it is impossible to establish a direct causal link between greenhouse gas accumulation and individual, relatively short-term climatic events, it is certain that we have been experiencing increasing numbers of climatic events unprecedented in the human experience. It is worth noting that the reduced sea ice cover of the Arctic Ocean, the retreat of mountain glaciers, reduced ice sheets in Greenland and West Antarctica, increased droughts and fires, increased severity of storms and flooding have all occurred with a warming of only 0.75°C (1.3°F). It is also certain that many of the greenhouse gases, including carbon dioxide, nitrogen, and methane, have lengthy residence times in the atmosphere, and that we will continue to be affected for years or even centuries to come by the atmospheric burden we are creating today.

Scientific Consensus Concerning Climate Change

Scientists are agreed about the reality of climate change and the underlying anthropogenic causes. Scientific investigation now focuses on what the effects of climate change will be.

smokestacks2While the concentrations of almost all greenhouse gases have been increasing since the Industrial Revolution, carbon dioxide has had the greatest effect on changing the climate. During the 1980s, humans released 5.5 billion tons of carbon (as carbon dioxide) into the atmosphere annually by burning fossil fuels (coal, oil, natural gas) for heat, transportation, and electricity. An additional 1.6 billion tons was released from anthropogenic (human-induced) changes in land-use (i.e. clearing land for agriculture, pastures, etc.) mostly through deforestation in the tropics. By the 1990s the average release of carbon dioxide from fossil fuels was 6.6 billion tons of carbon per year; and in the period 2000-2008, it was 7.7 billion tons carbon per year. In 2008, 8.7 billion tons carbon were released to the atmosphere from the combustion of fossil fuels. Emissions of carbon from deforestation have not increased since the 1980s, and may have decreased slightly.

Where does that annual release of carbon go? Approximately 4 billion tons of carbon per year are accumulated in the atmosphere. Ocean modelers find that the oceans take up approximately 25% of emissions per year (2.3 billion tons), and the land takes up about 3 billion tons (or 33% of total emissions). These flows or “fluxes” within the Global Carbon Cycle may be summarized using the formula:

Atmospheric increase = Emissions from fossil fuels + Net emissions from changes in land use – Oceanic uptake – Terrestrial carbon sink

Human beings are causing the release of carbon dioxide and other greenhouse gases to the atmosphere at rates much faster than the earth can cycle them. Fossil fuels – oil, coal, natural gas, and their derivatives – were formed through the compression of organic (once living) material for millions of years, yet billions of tons of these fuels are now being burned per year. The CO2 expelled into the atmosphere through these activities will remain in the atmosphere on the order of decades to centuries. This means that the CO2 emitted today will likely be affecting the climate for generations.

Despite the widespread recognition of this fact, worldwide emissions of fossil fuels have continued to grow at an ever increasing rate (Le Quéré et al., 2009). Emissions will increase even further as the developing world moves towards greater industrialization. In 2007, China passed the United States in being the number one emitter of carbon dioxide, though the United States still leads in terms of per capita emissions. Based on existing demographic, economic, social, and political conditions and trends, energy-related emissions of carbon dioxide are projected to increase from 7.9 billion tons carbon in 2006 to 11.0 billion tons carbon in 2030. Under business-as-usual scenarios, energy-related emissions of carbon from OECD countries are predicted to increase by 7 percent during this period, while the increase in emissions from non-OECD countries are predicted to increase by 68 percent (EIA, 2009). These emissions trajectories could be altered drastically, however, with changes to the drivers of emissions, such as economic growth and climate change mitigation strategies.

icelandIf greenhouse gases continue to be emitted into the Earth’s atmosphere at roughly the current rates, the resulting consequences will have far-reaching impacts:

The average surface temperature of the planet is expected to increase by about 0.2°C (0.3°F) per decade (IPCC 2007), reaching from 1.8°C to 4.0°C (3.2°F to 7.2°F) by the end of this century. Climate models estimate that increases in temperature will raise sea level between 0.28m and 0.42m by the end of the century, relative to the 1980-1999 mean sea level. But these estimates are conservative, as they are based on rates of ice flow from Greenland and Antarctica observed from 1993 to 2003. Other processes that affect ice flow were not included in the models, and more recent observations suggest that warming could increase the vulnerability of the ice sheets, increasing future rates of sea level rise above projections.

Warming tends to reduce the uptake of carbon by the oceans and terrestrial ecosystems, thereby increasing the fraction of carbon emissions that remain airborne and, thus, increasing the rate of CO2 increase in the atmosphere and the rates of warming above those projected by models.

Because of the intensification of the hydrological cycle with warming, heavy precipitation events will become more frequent, and future hurricanes and typhoons will be more intense. Snow cover is expected to contract; permafrost to thaw; and sea ice to shrink. Furthermore, the warming will not be evenly distributed over the surface of the earth but greater at high latitudes. Because as much as a third of the world’s terrestrial carbon is stored in the soils and peats of these high-latitude systems, the increased temperatures and permafrost thawing have the potential to release large quantities of CO2 to the atmosphere. This positive feedback could reverse the natural terrestrial sink that has prevailed over the last decades.

The increasing concentrations of carbon dioxide will also continue to increase the acidification of the oceans.

In short, the average global warming of 0.75°C (1.3°F) since the late 1800s has already increased the frequency of droughts, fires, floods in different parts of the world, increased the number and intensity of heat waves, and contributed to the spread of infectious diseases. To prevent further climatic disruption, including reduced productivity of food crops, emissions must not be allowed to increase, or even to remain constant. They must be reduced dramatically and quickly. The effects of the warming already observed indicate that an average warming of 2.0°C (3.6°F) is too much. The concentration of CO2 in the atmosphere must not be allowed to increase above 400 ppm and should be restored to 350 ppm or less.

ACIA, Impacts of a Warming Arctic: Arctic Climate Impact Assessment. Cambridge University Press, 2004. www.acia.uaf.edu.

IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Le Quéré, C., M.R. Raupach, J.G. Canadell, G. Marland, L. Bopp, P. Ciais, T.J. Conway, S.C. Doney, R.A. Feely, P. Foster, P. Friedlingstein, K. Gurney, R.A. Houghton, J.J. House, C. Huntingford, P.E. Levy, M.R. Lomas, J. Majkut, N. Metzl, J.P. Ometto, G.P. Peters, I.C. Prentice, J.T. Randerson, S.W. Running, J.L. Sarmiento, U. Schuster, S. Sitch, T. Takahashi, N. Viovy, G.R. van der Werf, and F.I. Woodward. 2009. Trends in the sources and sinks of carbon dioxide. Nature GeoScience 2:831-836.

Information Unit for Conventions (IUC) United Nations Environment Programme (UNEP). Understanding Climate Change: A Beginner’s Guide to the UN Framework Convention.