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.

What Is Climate Change?

Understanding the greenhouse effect is the first step to understanding how climate change is affecting our planet. Solar radiation interacts with the surface of the Earth in several forms: a portion of incoming solar energy is reflected back into space by the Earth’s atmosphere; another portion is dispersed and scattered by molecules in the atmosphere; and a large portion penetrates through Earth’s atmosphere to reach the planet’s surface. The radiation reaching Earth’s surface is largely absorbed, resulting in surface warming.


Much of this absorbed energy is later emitted back from the Earth as heat, and as it leaves the Earth, it again interacts with the atmosphere. Some of this energy also escapes back into space, but much of it is reflected back to the Earth’s surface yet again, by the molecules in the Earth’s atmosphere. These molecules also absorb some of the heat, and this occurrence is similar to the warming that occurs in an automobile parked outside on a sunny day. The contained atmosphere in the vehicle heats up much more than the environment outside the vehicle.


What Are Greenhouse Gases?

The molecules responsible for trapping energy in the Earth’s atmosphere are called greenhouse gases because they act somewhat like the glass in a greenhouse—retaining heat. 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 degrees Fahrenheit less than it currently is—permanently below freezing.


This satellite photo shows the atmosphere as an insulator above the Amazon rainforest in South America. Image Credit: NASA

Greenhouse gases act like a blanket around the planet, keeping the heat in. Increasing the concentration of these gases in the atmosphere increases the thickness of this insulator, thereby increasing the atmosphere’s ability to block the escape of heat. Too great a concentration of greenhouse gases can have dramatic effects on climate, and significant repercussions for Earth. Too low a concentration can have equally dramatic effects. Climates suitable for human existence are limited above a minimum threshold level of greenhouse gas concentration—one that makes 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 for the last few hundred thousand years. 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 trend, rising rapidly from 280 parts per million (ppm) in the early 1800s to a current level of 408 ppm, as of April 2016. The current concentration is 45 percent higher than it was at the start of the Industrial Revolution, and the global temperature climb has corresponded closely to that.


Source: NOAA

The Intergovernmental Panel on Climate Change (IPCC)

Noting these trends, and recognizing the potential for dramatic changes in the climate because of 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 IPCC reviews existing and developing peer-reviewed scientific literature to form an objective evaluation about the risk of human-induced climate change.

Their models predicted a rise of one to five degrees Celsius (two to 11.5 degrees Fahrenheit) in the global mean surface temperature during the next century, with sea-levels expected to rise by between seven and 23 inches (excluding possible future rapid changes of ice flow) by 2100 (IPCC 2013).


Source: IPCC

Improved Models

The Fifth Assessment Report of the IPCC, released in 2013, added weight to the connections between rising temperatures and continued greenhouse accumulations. For example, recorded global temperature change can be compared with computer models that predict temperature change under different external influences on the planet’s radiation. Known in the scientific world as “forcings,” these may include greenhouse gases, aerosols, solar radiation and other agents. Models using only natural influences fail to match the observed record of temperature anomalies since 1900. But the combination of natural and human impact models, produces a close match to the observed data. Climate models help reveal a clear “thumbprint” of our impacts on the planet.

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, along with the rate of atmospheric CO2. 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). The 1990s was the warmest complete decade since 1850, and was, on average, 0.43 degrees Celsius warmer than the period 1961-1990.

Other Impacts

Glaciers are present on every continent other than Australia, and they function as reasonably well-distributed indicators of changing global temperatures. Worldwide, glaciers and ice fields 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 examples.


Image source: National Snow and Ice Date Center

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 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 decreases in sea ice, increasing precipitation and river discharge, thawing of glaciers and permafrost, and changes in plant and animal abundances and distributions.

It is impossible to establish a direct causal link between greenhouse gas accumulation and individual, relatively short-term climatic events, but it is certain that we have been experiencing increasing numbers of climatic events unprecedented in human experience. 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, and increased severity of storms and flooding have all occurred with a warming of only 0.75 degrees Celsius or 1.3 degree Fahrenheit


Click image to enlarge. Source: NOAA

Scientific Consensus Concerning Climate Change

Scientists are agreed about the reality of climate change, and that it is caused by humans. Our scientific investigation now focuses on what the effects of climate change will be.

How It All Adds Up

While 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, and natural gas for heat, transportation, and electricity. An additional 1.6 billion tons was released from human-induced changes in land-use (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 of carbon per year. In 2008, 8.7 billion tons of carbon were released to the atmosphere from the combustion of fossil fuels. Emissions of carbon from deforestation were stable from 1980 to 2014, but since then, there has been a significant increase.

Where does that annual release of carbon go? Approximately four billion tons of carbon per year are accumulated in the atmosphere. Ocean modelers find that the oceans take up approximately 25 percent of emissions per year (2.3 billion tons), and the land takes up about three billion tons (or 33 percent of total emissions). These 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


This diagram of the carbon cycle shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. (Diagram adapted from U.S. DOE, Biological and Environmental Research Information System by NASA Earth Observatory.)

Humans cause 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 each year. The CO2 expelled into the atmosphere through these activities will remain in the atmosphere for decades to centuries. This means that the CO2 emitted today will likely be affecting the climate for generations to come.

Despite widespread recognition of this, worldwide emission of fossil fuel continues to grow at an ever increasing rate (Le Quéré et al., 2009). Emissions will increase even more 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.


Leading global emissions. Source: Carbon Brief, 2012


Tianjin Power Plant, China. Photo courtesy YaoHuo2k7.

Based on existing demographic, economic, social, and political conditions and trends, energy-related emissions of CO2 are projected to increase from 7.9 billion tons of carbon in 2006 to 11.0 billion tons of carbon in 2030. Under business-as-usual scenarios, energy-related emissions of carbon from the Organisation for Economic Co-operation and Development (OECD) countries are predicted to increase by seven 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 improvements to the drivers of emissions, such as economic growth and climate change mitigation strategies.

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

Increasing Mean Sea Level – The global temperature 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 these increases 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.

Affects of Warming – Warming tends to reduce the uptake of carbon by the oceans, and ecosystems on the land. This in turn elevates the rate of CO2 increase in the atmosphere along with the subsequent rates of warming above those projected by the models.

More Intense Rains, Hurricanes and Typhoons – Because of the intensification of the hydrological cycle with a warming planet, floods will become more frequent, and hurricanes and typhoons will be more intense. Snow cover is expected to contract; permafrost to thaw; and sea ice to shrink. And the warming will not be evenly distributed over the surface of the earth—it will be greater at high latitudes.


Changes in the hydrological cycle common to the U.S. East and West coasts. Courtesy EPA

Permafrost Thaw – Because as much as a third of the world’s terrestrial carbon is stored in the soils and peats of the Arctic region, the increased temperatures and permafrost (permanently frozen soils) thawing have the potential to release large quantities of CO2 to the atmosphere. This feedback could reverse the natural land-based uptake of CO2 that has prevailed over the last decades. The increasing concentrations of carbon dioxide will also continue to increase the acidification of the oceans.

In Summary – Once CO2 is added to the atmosphere, it stays in the atmosphere for centuries, and the warming effects last even longer. Because of this, warming is proportional to cumulative human greenhouse gas emissions. We have already emitted so much it is unlikely that we can limit warming to two degrees Celsius without removing large amounts of CO2 from the atmosphere. The only way to do this at the scale required is through reforestation.

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.

A New Strategy

Forests: The bridge to a fossil-free future.

Key Science – Making It Easier

Improved management of tropical forests could stabilize or even reduce the concentration of carbon dioxide in the atmosphere. This would provide a bridge to a planet powered by renewable energy. Restoration of tropical forests would also make it much easier to achieve the goal of limiting global warming to two degrees Celsius.

Most strategies for meeting the two-degree goal rely on immediate and severe reductions in fossil fuel use, which would be difficult to achieve. However, if tropical deforestation were stopped and even reversed, enough carbon dioxide could be removed from the atmosphere to stabilize concentration at current levels, while the world reduced its dependence on fossil fuels. This is not a complete solution for curbing climate change, but it would be an important step. Compared to continuing the present rates of deforestation and forest degradation, a scenario of aggressive management of tropical forests would increase by 10-15 years the time available to eliminate fossil fuel use and still be likely to limit global warming to two degrees Celsius. This would provide a much needed window of opportunity to wean the planet off fossil fuels.

WHRC scientists and their colleagues emphasize that forest restoration is not a substitute for reducing fossil fuel use. This tropical land management can only work in tandem with the elimination of fossil fuel combustion. While not a complete solution, reforestation can be the bridge to a world with 100 percent renewable energy from sources like the wind and sun.


Stopping the destruction of the great forests of our world, and reducing our impact on their ability to function as healthy ecosystems could reduce emissions more than commonly thought. Allowing forests to continue growing would maintain the current carbon sinks in these forests—areas that naturally absorb carbon dioxide—and it would play a critical role in soaking up greenhouse gas emissions. This would also help maintain the conditions needed for sustainable agriculture, ecosystems, and water supplies.

Doing the Math – Carbon-smart forest management

To have a 75 percent chance of limiting global warming to two degrees Celsius, total emissions of carbon cannot exceed 1.2 billion tons (Meinshausen et al., 2009). One ton is approximately the same weight as two average sized cars. That limit applies to total cumulative discharge from fossil fuel use, which includes land use like deforestation. In effect, minimizing emissions from deforestation would allow additional emissions from fossil fuel use. This carbon-smart forest management can buy additional time to eliminate the use of fossil fuels. Below are two scenarios for doing this:

  1. At present, tropical forests release a net of about 1.2 billion tons of carbon into the atmosphere each year because of deforestation and forest degradation. These forest-based emissions “use up” some of the 1.2 billion tons we are able to emit while still being likely to limit warming to two degrees Celsius. If these emissions continue unabated, we would need to ramp down fossil fuel use within 19-20 years in order to meet our goal.
  2. By contrast, management of tropical forests can help meet the two-degree goal by providing “negative emissions” for a limited time. The present net emissions from tropical forests of 1.2 billion tons per year is the difference between gross emissions of 1.7 billion tons per year from forest loss, and absorption of 0.5 billion tons per year through regrowth of previously deforested areas. In other words, stopping forest loss, for this equation, would see tropical forests absorbing 0.5 billion tons per year. At the same time, reforesting as many as 500 million football fields of formerly forested land would remove another 1 billion tons each year. Under this scenario tropical forests could potentially absorb as much as 1.5 billion tons of carbon per year.


The annual emissions of carbon to, and removals from, the atmosphere in tropical regions as a result of forest management. Both emissions and removals have generally increased over time as forests have been cleared for agricultural lands. Emissions result from deforestation and forest degradation; removals result from forest growth following the harvest of wood and agricultural abandonment. The spikes in emissions since the late 1990s result from the burning of peatlands in Southeast Asia as forests are cleared for oil palm plantations.

With these forest-management policies in place, the world could ramp down fossil fuel use over 33 years, and still be likely to limit global warming to two degrees Celsius. This is 10-15 years longer than the first scenario.

Proper forest management is the only climate change alleviation technology that is:

  • available immediately;
  • capable of providing negative emissions at the necessary scale; and
  • proven to have additional benefits for the local and global climate.

Trees are the mechanism by which carbon dioxide can be removed from the atmosphere. We need to reforest our landscapes.


  • Transition from fossil fuels to renewables within the next few decades, and use forest on land to store carbon. This will limit the growth of carbon dioxide in the atmosphere during this transition.
  • Support policy frameworks such as the New York Declaration of Forests, and REDD+ economic incentives to reduce deforestation.
  • Identify areas with the greatest potential for accumulating more carbon on land, and focus restoration efforts there. Alert decision makers to the potential for their jurisdictions to help remove carbon from the atmosphere through land management.
  • Measure progress in meeting goals for increasing land carbon storage using satellite data and other means.


Scholarly References 

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.

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

Understanding and Attributing Climate Change – IPCC makes the case with this reference text.

Assessing key vulnerabilities and the risk from climate change – a reference text by the IPCC.


Scholarly Articles

Nature Climate Change 5, 27–36 (2015) doi:10.1038/nclimate2430
Received 31 July 2014; Accepted 10 October 2014; Published online 18 December 2014.


General Audience Books

A University of Wisconsin list of books about climate change positioned for young readers.

A University of Wisconsin list of climate change books focused on economies and society.

A University of Wisconsin list of climate change books on water.

A review of the scholarly book Global Food Futures: Feeding the World in 2050, by Brian Gardner.

A review of a book that offers a comprehensive analysis of unique challenges that developing countries face in the domain of climate change: Climate Governance in the Developing World, edited by David Held et al.

A link to the website of the authoress of the book, This Changes Everything: Capitalism VS. The Climate. “Before engaging with environmental issues, Klein was part of the movement against neoliberal globalization. Her new book picks up a central critique of many climate groups, explaining why capitalism is deeply related to the climate crisis. As a member of the board of 350.org, she shares not only comment but practical insights into the strategies and plans of the movement,” writes Matthias Dietz of The Guardian.

The End of Nature, by US-scholar and long-term climate activist Bill McKibben, is said to be the first non-scientific book on global warming. It treats climate change as one part of a global ecological crisis, which can only be solved by a radical change of the perception, and treatment, of nature. The book is important because it demanded extensive change at an early stage (1989) and it directs this demand not only towards politicians, but to society as a whole,” writes Matthias Dietz of The Guardian.

Moral Ground: Ethical Action for a Planet in Peril by Kathleen Dean Moore & Michael P. Nelson, Editors. Foreword by Desmond Tutu. Do we have a moral obligation to take action to protect the future of a planet in peril? This book shares the testimony and responses of over 80 visionaries who answered that question from their perspectives and in a variety of ways. Contributors include theologians and religious leaders, naturalists, scientists, elected officials, business leaders, activists and writers who share their thoughts and moral visions for environmental repair and sustainability.


General Media Articles

Satellite alerts track deforestation in real time: System uses Landsat data to issue warnings just hours after tree loss is detected. News Brief in Nature Magazine.

Stopping deforestation: Battle for the Amazon – Brazil has waged a successful war on tropical deforestation, and other countries are trying to follow its lead. But victory remains fragile. Nature Magazine News Feature.

No, Tropical Deforestation Rates Aren’t Falling: Contrary to earlier reports, Earth’s rainforests are more at risk than ever before. Discover Magazine News Brief.

UN Report: Tropical Deforestation is a Booming Business for Organized Crime. Discover Magazine Blog.

The BBC examines a powerful tool in understanding global warming – computer models.

Grist Magazine’s “How to Talk to a Climate Skeptic: Responses to the most common skeptical arguments on global warming” – a great resource.



On July 13, the UK’s Foreign Commonwealth Office commissioned this independent report authored by a range of military, finance, science and energy experts. It argues that climate change is a risk that should be treated as other risks to national and international security, such as nuclear proliferation and terrorism.


Descriptions of Climate Change

NASA’s basic description of climate change for Grade K4.

The World Wildlife explanation of climate change with a National Geographic video of Al Gore’s explanation of the phenomena.

The EPA’s description of why climate change is happening.



The Society for Environmental Journalists “Essential Reading List” for climate change reporters.

The Library of Congress Science’s Reference Services reading list for Earth’s water cycle and climate change.