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Future Temperature Changes
Climate Feedbacks | Temperature Change Projections | The Earth's Commitment to Warming
Most climate change scenarios project that greenhouse gas concentrations will increase through 2100 with a continued increase in average global temperatures (IPCC 2007). How much and how quickly the Earth's temperature will increase remains unknown given the uncertainty of future greenhouse gas, aerosol emissions and the Earth's response to changing conditions. In addition, natural influences, such as changes in the sun and volcanic activity, may affect future temperature, although the extent is unknown because the timing and intensity of natural influences cannot be predicted.
Advancements in model simulations, combined with more data on observed changes in climate, have led to increased confidence in projections of future temperature changes. In its 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC) for the first time was able to provide best estimates and likely ranges for global average warming under each of its emissions scenarios.
Climate Feedbacks
Future temperature changes will not depend solely on the direct effects of natural and human influences. The temperature may also change due to what are known as climate "feedbacks" – the climate system's responses to these direct effects. These feedbacks can increase or decrease the direct effect:
- An example of a positive feedback that could arise from warming results from melting ice and is known as the ice-reflectivity feedback. If temperatures warm near the Arctic, sea ice would likely melt. Because seawater is not as reflective as ice, the loss of ice would result in additional warming (since the ocean would absorb more solar radiation than ice).
- An example of a negative feedback that could arise from warming is an increase in low clouds from increased evaporation (which warming promotes). The addition of low and/or thick clouds (e.g. stratus, cumulonimbus clouds) would tend to cool the climate (by reflecting sunlight) – decreasing the warming.
Many other feedbacks (both positive and negative)
resulting from increasing greenhouse gas concentrations will likely influence
the rate of warming. The net effect of known feedbacks considered in most
future warming projections
is almost certainly positive, but there is considerable uncertainty about
the exact magnitude. More information on climate change feedbacks can be
found in the National Academy of Sciences report: “Understanding
Climate Change Feedbacks.” 
Temperature Change Projections
Due to uncertainties about future emissions and concentrations of greenhouse gases, their net warming effect in the atmosphere, and the response of the climate system, estimates of future temperature change are uncertain. With these caveats in mind, the IPCC made the following projections of future warming (IPCC, 2007):
- The average surface temperature of the Earth is likely to increase by 2 to 11.5°F (1.1-6.4°C) by the end of the 21st century, relative to 1980-1990, with a best estimate of 3.2 to 7.2°F (1.8-4.0°C) (see Figure 1). The average rate of warming over each inhabited continent is very likely to be at least twice as large as that experienced during the 20th century.
- Warming
will not be evenly distributed around the globe (see Figure 2):
- Land areas will warm more than oceans in part due to water's ability to store heat.
- High latitudes will warm more than low latitudes in part due to positive feedback effects from melting ice (as discussed above).
- Most of North America; all of Africa, Europe, northern and central Asia; and most of Central and South America are likely to warm more than the global average. Projections suggest that the warming will be close to the global average in south Asia, Australia and New Zealand, and southern South America.
- The warming will differ by season, with winters warming more than summers in most areas.
- For additional explanatory information about some of the projected spatial and seasonal differences in warming, see the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) fact sheet "Patterns of Global Warming" (PDF, 1 pp., 15 KB, About PDF)
Figure 1: Temperature projections to the year 2100, based on a range of emission scenarios and global climate models. Scenarios that assume the highest growth in greenhouse gas emissions provide the estimates in the top end of the temperature range. The orange line (“constant CO2”) projects global temperatures with greenhouse gas concentrations stabilized at year 2000 levels. Source: NASA Earth Observatory, based on IPCC Fourth Assessment Report (2007)
Figure 2: Projected future regional patterns of warming based on three emissions scenarios (low, medium, and high growth). Source: NASA Earth Observatory, based on IPCC Fourth Assessment Report (2007)
The Earth's Commitment to Warming
According to several recent studies, even if the composition of today's atmosphere was fixed (which would imply a dramatic reduction in current emissions), surface air temperatures would continue to warm (by up to 1.6ºF or 0.9 ºC); see the “constant CO2” line in Figure 1 (IPCC, 2007). The studies suggest that a portion of the warming associated with past human activity has not yet been realized due to heat being stored in the ocean, and that the Earth is committed to continued warming. In addition, many of the greenhouse gases that have already been emitted remain in the atmosphere for decades or longer, and will continue to contribute to warming for their duration. For more information, see the National Oceanic and Atmospheric Administration's (NOAA) GFDL fact sheet: Global Warming Commitment: Temperatures Would Rise Even With No Additional Greenhouse Gas Increases (PDF, 1 pp., 85 KB, About PDF). Also see this research update from NASA's Goddard Institute for Space Studies (GISS): Earth's Energy Out of Balance as well as the article "The Earth's Big Heat Bucket" from NASA's Earth Observatory.
References
- IPCC, 2007: 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 (eds.)].
- National
Research Council (NRC), 2002: Abrupt Climate Change, Inevitable Surprises.
National Academy Press, Washington, DC. National Academy Press,
Washington, DC