Climate Basics

Climate Change and...

Climate Change Primer

Current Climate Change

Global Warming and Rising CO2

Global average surface temperatures have increased markedly over the last century (Figure 1). Humans have been measuring temperature directly since the mid- 1800's; these measurements show that temperature increased by 1.33°F (0.74°C ) between 1906 and 2005, and that the rate of warming is increasing (IPCC 2007a Ch.3). The decade from January 2000 to December 2009 was the warmest decade during this time period (NASA 2011). Although 1.33°F may not seem like a large temperature change, on a global scale this has huge implications for many of the earth's processes that affect ecosystems and humans. To put the number in perspective, many scientists think that temperature increases in excess of 3.6°F (2.0°C) relative to 1980-1999 will result in 'dangerous' climate change; others say that even lesser increases would be enough to create outcomes dangerous to human civilization (Anderson & Bows, 2011).

Global Temperature Anomalies

Figure 1 - Global temperature trend from 1880 to present, compared to a base period of 1951-
1980. Global temperatures continue to rise, with the decade from 2000 to 2009 as the warmest on
record Source: NASA/Earth Observatory/Robert Simmon

Excess greenhouse gases in the atmosphere are a measureable and significant contributor to global warming, and their concentrations have steadily increased over the past century (IPCC 2007). Carbon dioxide (CO2) , the most important greenhouse gas in terms of climate change, has been measured directly since 1958. Additionally, atmospheric levels of CO2 can be reconstructed for hundreds of thousands of years into the past using methods such as analyzing air bubbles trapped in ice. CO2 concentration in late 2011 was at 391 parts per million, a level that is higher than at any point during the past 800,000 years (Global Carbon Project 2011; Figure 2). Growth rates of atmospheric CO2 are still high, with 2010 experiencing one of the largest annual growth rates of the past decade (Global Carbon Project 2011).

Trends in Atmospheric CO<sub>2</sub> and Global Surfacec Temperature

Figure 2 - Human society is entering uncharted territory as atmospheric levels of greenhouse gases
continue to rise. Today's carbon dioxide levels are substantially higher than anything that has occurred
for more than 800,000 years (last 400,000 years pictured here). Source: Center for Climate and Energy

Climate Change

For an animated look at how CO2 concentrations have changed over the last 800,000 years, see this video created by the NOAA Earth System Research Laboratory

Rising global temperatures are causing the Earth's climate patterns to change. Climate can be defined as the "average weather," or the average long-term (multi-decadal) meteorological conditions and patterns for a given area. Changes in climate that are occurring as the planet warms include seasonal and regional changes in temperature and precipitation, (USGCRP 2009, IPCC 2007a Ch.3), and increasing extreme weather events (IPCC 2011). As an example, precipitation from 1900 to 2005 increased significantly in some areas (eastern parts of North and South America, northern Europe and northern and central Asia), and declined in other regions during the same time period (the Sahel, the Mediterranean, southern Africa and parts of southern Asia - IPCC 2007a Ch.3).

In conjunction with climate change, during the 20th century there has been a nearly worldwide reduction in glacial mass and extent, a decrease in snow cover in many Northern Hemisphere regions, a decrease in Arctic sea ice thickness and extent, a decrease in the length of river and lake ice seasons, permafrost warming (IPCC 2007a Ch.4), warmer ocean temperatures, and rising sea levels (IPCC 2007a Ch.5), among other observed changes (Figure 3).

Figure 3

Figure 3 - Observed changes in temperature, sea level, and Northern Hemisphere
snow cover for March-April. All differences are relative to averages for the period 1961-1990. The shaded areas
represent the possible range. Source: IPCC 2007b

For up-to-date information on temperature, carbon dioxide, and other indicators of a warming planet, see the NASA Global Climate Change - Key Indicators page

Climate Mechanisms

The Greenhouse Effect

The physical mechanisms that cause greenhouse gases to warm the planet, commonly known as the 'greenhouse effect', are well understood and were scientifically demonstrated beginning in the mid-1800s (Tyndal 1861). Of the solar energy that is directed toward Earth, about 30% is reflected back to space by clouds, dust, and haze (Ramanathan & Feng 2009). The remaining 70% is absorbed by the atmosphere and the Earth's surface. The Earth's warmed surface releases some of that absorbed energy as infrared radiation, a form of light, but invisible to human eyes. Greenhouse gases in the atmosphere including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor, absorb this infrared radiation and keep it from passing out into space. This energy is then reradiated in all directions, and the energy that is directed back toward the Earth warms the planet.

Figure 4

Figure 4 -An idealized model of the greenhouse effect. Source: IPCC 2007a Ch.1

Human Influence on Greenhouse Gases

Without the natural presence of energy-absorbing greenhouse gases in the Earth's atmosphere, the average temperature at Earth's surface would be below the freezing point of water (IPCC 2007a Ch.1). However human activities have led directly to increases in greenhouse gas concentrations and therefore an enhanced greenhouse effect.

The 2007 United Nations Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) provides the most substantive and authoritative evaluation of human-caused global climate change to date. According to this report, "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely [> 90% probability] due to the observed increase in anthropogenic greenhouse gas concentrations." (IPCC 2007b). Independent studies using a variety of methods strongly corroborate this conclusion (e.g. Lean 2010, Huber & Knutti 2011). Examinations and model simulations of many possible explanations of global warming show that we can only explain the strong temperature increases of the past 120 years if we account for human influences (e.g. Figure 5).

Figure 5 - Contributions of to the average monthly global surface temperatures by individual ENSO [El Niño], volcanic, solar, and human-caused influences. Source: Lean 2010

Human activity has had the most notable impact on carbon dioxide concentrations, which as noted earlier, have increased dramatically (Figure 2), mainly through fossil fuel burning, cement production, and deforestation. Methane, another potent greenhouse gas, is emitted by activities such as rice and livestock agriculture and biomass burning, and is currently at its highest concentrations of the past 650,000 years (IPCC 2007a Ch.2). Nitrous oxides have increased due in part to agricultural fertilization and fossil fuel burning; other gases emitted from industrial processes, such as halocarbons, also play a role in warming (Figure 5). Many of these greenhouse gases are likely to reside in the atmosphere for decades to centuries (CDIAC 2011). The most abundant greenhouse gas is water vapor, but water vapor is short lived in the atmosphere (on the order of days) and is dependent on temperature. So, human activities have little direct influence on water vapor, although human-caused warming can increase water vapor concentrations and amplify the warming effect (Held and Soden 2000).

Radiative forcing of climate between 1750 and 2005

Figure 6 -The amount of warming influence (red bars) or cooling influence (blue bars) that different
factors have exerted on the Earth's climate over the industrial era (from 1750 – 2005). A longer bar
signifies a greater influence. Source: IPCC 2007a Ch.2

Natural Climate Cycles

Climate varies without human influence, and this natural variation is a backdrop against which human- caused climate change occurs. These patterns hold important lessons for understanding the magnitude and scope of current and future climate changes.

Cyclical variations in the Earth's climate occur at multiple time scales, from years to decades, centuries, and millennia. Cycles at each scale are caused by a variety of physical mechanisms. Climate over any given period is an expression of all of these nested mechanisms and cycles operating together.

Millennial Climate Cycles

Major glacial (cold) and interglacial (warm) periods are initiated by changes in the Earth's orbit around the Sun, called Milankovitch cycles. These cycles have occurred at different intensities on multi- millennial time scales (10,000 – 100,000 year periods). The orbital changes occur slowly over time, influencing where solar radiation is received on the Earth's surface during different seasons (IPCC 2007a Ch.6).

By themselves, these changes in the distribution of solar radiation are not strong enough to cause large temperature changes. However they can initiate powerful feedback mechanisms that amplify the slight warming or cooling effect caused by the Milankovitch cycle. One of these feedbacks is caused through changes in global surface reflectivity (also called albedo). Even a slight increase in solar radiation at northern latitudes can increase ice melt. As a result, less sunlight is reflected from the bright white surface of the ice, and more is absorbed by the Earth, increasing overall warming. A second feedback mechanism involves atmospheric greenhouse gas concentrations, such as carbon dioxide. The slight warming initiated by changes to Earth's orbit warms oceans, which allows them to release carbon dioxide. As we've seen, more carbon dioxide in the atmosphere causes more warming, creating an amplifying effect (Hansen 2003). Distinct feedbacks in atmospheric CO2 concentrations may lag warming or cooling caused by orbital changes by as much as 1000 years.

In this way, what begins as fairly minor changes in orbit can produce the glacial and interglacial cycles of the last 800,000 years. A major concern with current climate change is that similar feedback mechanisms will cause a 'runaway' warming effect in modern times that will be extremely difficult to halt or reverse.

Century-scale Climate Cycles

In addition to multi-millennial glacial and interglacial cycles, there are shorter cold-warm cycles that occur on approximately 200 to 1,500 year time scales. The mechanisms that cause these cycles are not completely understood, but are thought to be driven by changes in the sun, along with several corresponding changes such as ocean circulation patterns (Bond et al. 2001, Wanner et al. 2008). The Medieval Warm Period (900-1300 AD) and the Little Ice Age (1450 to 1900 AD) are examples of warm and cold phases in one of these cycles. Some of these cycles, such as the Medieval Warm Period, may be regional, not necessarily reflecting large changes in global averages. Understanding and reconstructing the regional patterns of climate change during each of these periods is considered very important in accurately analyzing future regional impacts such as drought patterns (Mann et al. 2009).

Interannual to Decadal Climate Cycles

Ocean-atmosphere interactions regularly cause climate cycles on the order of years to decades. One of the most well-known cycles is the El Niño-Southern Oscillation (ENSO), an interaction between ocean temperatures and atmospheric patterns (commonly known as El Niño or its opposite effect, La Niña). ENSO events occur every 3 to 7 years, and bring different conditions to different parts of the world (IRI 2007). For example, in the U.S., El Niño events can result in dry weather in the pacific northwest and southeast, but wet weather in the southwest (IRI 2008).

Many other cyclical changes due to oceanic and/or atmospheric processes have been described, such as the Pacific Decadal Oscillation (PDO) which occurs in cycles of 25-45 years, and the Atlantic Multi- decadal Oscillation (AMO), occurring on approximately 65-85 year cycles (Deser et al. 2010). It is not yet well known how each of these reoccurring cycles might interact with the enhanced greenhouse effect.


For more information about natural climate cycles and their implications, see a presentation by paleoecologist Connie Millar.

Natural climate cycles can help to understand what climate patterns are expected, and how the recent increase in greenhouse gas emissions is causing deviations from these expected patterns. They can offer insight into amplifying effects that may exacerbate warming as greenhouse gas concentrations rise (Wolff 2011). They may also provide insight on regional impacts of climate change, which will be enormously important for developing adaptation strategies for human and ecological communities. However, it is important to recognize that current rates of global climate change are extremely rapid compared to past changes (IPCC 2007a Ch.6), and may produce conditions that have not been anticipated.:

Expected Effects of Climate Change

Temperature and Precipitation Projections

Global average temperatures are projected to rise over this century. Temperature increases will vary regionally and seasonally; for example, temperature increases at polar latitudes are expected to be greater than increases near the equator (IPCC 2007a Ch.11). Part of this future warming is inevitable due to the long-lived greenhouse gases that are already present in Earth's atmosphere. However the full extent of warming will depend in part on future emissions of greenhouse gases. The IPCC has developed a range of 'emissions scenarios' to describe several plausible futures that depend on factors like population, economic growth, technological advances, and others. These are frequently used by climate modelers in projecting the ranges of future climate (IPCC 2000). New emissions scenarios are in development and will be used in the upcoming 2013-2014 version of the IPCC report. Average temperatures in the U.S. over the next century are expected to increase by approximately 7 to 11°F (3.9 to 6.1°C) under a higher IPCC emissions scenario and by approximately 4 to 6.5°F (2.2 to 3.6°C) under a lower emissions scenario. (USGCRP 2009 – Figure 7). Both lower and higher temperature changes are possible, if future emissions fall below or above the IPCC emissions scenarios.

Higher Emissions Scenario Projected Temperature Change (F)

Figure 7 - Projected temperature changes in the U.S. for the current century under two IPCC emissions
scenarios. Source: USGCRP, National Climate Change

Precipitation changes will also vary seasonally and regionally, and are more uncertain than temperature changes. Models project that northern areas in the U.S. will generally become wetter, and southern areas, especially the southwest, will generally become drier (USGCRP 2009). In northern areas, a greater proportion of annual precipitation is expected in the winter and spring, and may fall as rain rather than snow due to warmer temperatures. In the southwest, increased evaporation due to higher temperatures may increase stress on water resources (USGCRP 2009). Across all areas of the United States, the amount of heavy precipitation is expected to increase, especially in the Midwest and northeast, following observed trends from 1958 to present day (Figure 8). Although modeled precipitation projections are improving, there is still a high degree of uncertainty and specific regional patterns could differ substantially from these general trends.

Projected Changes in Light, Moderate and Heavy Precipitation by the 2090's

Figure 8 - Heavy precipitation events are expected to increase over the next century, while the light
precipitation will likely be more infrequent. Projections are based on models used in the IPCC 2007
Report. USGCRP 2009, National Climate Change

Effects on Ecosystems and Ecosystem Processes

For overviews on regional climate change projections in the U.S. please see the USGCRP report: Alaska, Coasts, Great Plains, Islands, Midwest, Northeast, Northwest, Southeast, Southwest

The climate changes expected over the next century will have huge consequences for ecosystems and the benefits they provide, including the provision of wood and fuel, food, temperature and flood regulation, erosion control, recreational and aesthetic value, and species habitat, among others.

Climate changes are likely to affect important ecological processes that will in turn affect key natural resources. For example, temperature and precipitation changes have strong implications for water resources and hydrologic cycling. In addition, disturbances such as insects, wildfire, invasive plants, and plant diseases will become more frequent in some areas of the country. The emissions that cause climate change also lead to air quality problems that put additional stress on trees.

Coupled with altered hydrology and increased disturbance and stress, climate change will affect how vegetation is distributed within the U.S., and will cause changes for aquatic ecosystems, wildlife species and soils. How these resources are affected will have broad implications for maintaining ecosystem services, including biodiversity and the carbon storage capabilities of forests. Each impact on one aspect of an ecosystem can affect a variety of others, producing a series of cumulative effects that can make it difficult for ecosystems to adapt.

Meeting the diverse challenges that climate change is imposing on Earth's environments requires many approaches, and specific responses will depend heavily on the management goals for a particular resource. Scientists are currently working to understand the risks posed to ecosystems, through examining characteristics and changes in landscapes and conducting assessments on impacts and ecosystem vulnerabilities. Public lands, private lands, wilderness areas, and urban neighborhoods will all be affected, and each will require different management considerations. Specific management practices such as silviculture are potentially valuable tools for helping forests respond to a changing climate.

For those charged with managing ecosystems, climate change can seem like a daunting challenge. Fortunately, a range of management options exist to help ecosystems adapt to climate changes, and to contribute to climate change mitigation by reducing the amount of greenhouse gases in the atmosphere. These options are often complementary to actions that land managers employ regularly.

The majority of the CCRC is dedicated describing ecosystem responses to climate change, and how natural resource management may be able to respond to those changes. Please follow the links in the text, or explore the rest of the website for further information.

Need more information?

See the following primers and resources for more introductory information on climate change.

Climate Change Resource Center:

Center for Climate and Energy Solutions:
Facts and Figures
Climate Change 101 Series

United States Global Change Research Program:
Global Climate Change Impacts in the United States.

Climate Change Science Facts

NASA Earth Observatory
Global Warming - (navigate in right hand menu).
World of Change: Global Temperatures


Anderson A.; Bows, A. 2011. Beyond 'dangerous' climate change: emission scenarios for a new world. Philosophical Transactions of the Royal Society. 369: 20-44.

Bond, G.; Kromer, B.; Beer, J.; Muscheler, R.; Evans, M.; Showers, W.; Hoffmann, S.; Lotti-Bond, R.; Hajdas, I.; Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science. 294: 2130-2136.

Carbon Dioxide Information Analysis Center (CDIAC). 2011. Recent Greenhouse Gas Concentrations. (Accessed 12-8-2011)

Center for Climate and Energy Solutions (C2E). 2012. Long Term Trends in Carbon Dioxide and Surface Temperature. (Accessed 1-9-2012).

Deser, C.; Alexander, M.A.; Xie, S.P.; Phillips, A.S. 2010. Sea Surface Temperature Variability: Patterns and Mechanisms. Annual Review of Marine Science. 2: 115-143.

Global Carbon Project. 2011. Carbon budget and trends 2010. (Accessed 12-8-2011)

Hansen, J.E. 2003. Can we defuse the global warming time bomb? (Accessed 12-8-2011)

Held, I.M.; Soden, B.J. 2000. Water vapor feedback and global warming. Annual Review of Energy and the Environment. 25:441-475.

Huber, M.; Knutti, R. 2011. Anthropogenic and natural warming inferred from changes in Earth's energy balance. Nature Geoscience. Advance Online Publication.

Lean, J. 2010. Cycles and trends in solar irradiance and climate. Wiley Interdisciplinary Reviews: Climate Change. 1: 111-122.

The International Research Institute for Climate and Society (IRI). 2007. Overview of the ENSO System. (Accessed 12-8-2011)

The International Research Institute for Climate and Society (IRI). 2008. Global Effects of ENSO. (Accessed 12-8-2011)

Mann, M.E.; Zhang, Z.; Rutherford, S.; Bradley, R.S.; Hughes, M.K.; Shindell, D.; Ammann, C.; Faluvegi, G.; Ni, F. 2009. Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science. 27 (326): 1256-1260.

NASA – Goddard Institute for Space Studies. 2011. NASA Research Finds 2010 Tied for Warmest Year on Record. Research News.

IPCC, 2000: IPCC Special Report: Emissions Scenarios. Nakicenovic, N.; Swart, R. (Eds.) Cambridge University Press, Cambridge, UK. 570 pp.

IPCC, 2007a: 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.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.; Miller, H.L. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

IPCC, 2007b: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K.; Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp.

IPCC, 2011: Summary for Policymakers. In: Intergovernmental Panel on Climate Change, Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C. B.; Barros, V.; Stocker, T.F.; Qin, D.; Dokken, D.; Ebi, K.L.; Mastrandrea, M. D.; Mach, K. J.; Plattner, G.K.; Allen, S.; Tignor, M.; Midgley, P. M. (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA.

Ramanathan, V.; Feng, Y. 2009. Air pollution, greenhouse gases and climate change: Global and regional perspectives. Atmospheric Environment. 43: 37-50.

Tyndal J. 1861. On the absorption and radiation of heat by gases and vapours, and on the physical connexion of radiation, absorption, and conduction. Philosophical Magazine. 22:169–94, 273–85

United States Global Change Research Program (USGCRP). 2009. Global Climate Change Impacts in the United States. Karl, T.R.; Melillo, J.M.; Peterson, T.C. (eds). Cambridge University Press.

Wanner, H.; Beer, J.; Bütikofer, J.; Crowley, T.J.; Cubasch, U.; Flückiger, J.; Goosse, H.; Grosjean, M.; Joos, F.; Kaplan, J.O.; Küttel,M.; Müller, S.A.; Prentice, C.; Solomina, O.; Stocker, T.F.; Tarasov, P.; Wagner,M.; Widmann, M. 2008. Mid- to Late Holocene climate change: an overview. Quaternary Science Reviews. 27: 1791-1828.

Wolff, E.W. 2011. Greenhouse gases in the Earth system: a palaeoclimate perspective. Philosophical Transactions of the Royal Society. 369: 2133-2147.

bottom right