Predicting Future Warming

	As the world consumes ever more fossil fuel energy, greenhouse gas concentrations will continue to rise, and Earth’s average surface temperature will rise with them. Based on plausible emission scenarios, the IPCC estimates that average surface temperatures could rise between 2°C and 6°C by the end of the 21st century.
	At first glance, these numbers probably do not seem threatening. After all, temperatures typically change a few tens of degrees whenever a storm front moves through. Such temperature changes, however, represent day-to-day regional fluctuations. When surface temperatures are averaged over the entire globe for extended periods of time, it turns out that the average is remarkably stable. Not since the end of the last ice age 20,000 years ago, when Earth warmed about 5°C, has the average surface temperature changed as dramatically as the 2°C to 6°C change that scientists are predicting for the next century. Scientists predict the range of temperature increase by running different scenarios through climate models. Because scientists can’t say how human society may change over the next century, or how certain aspects of the climate system (such as clouds) will respond to global warming, they give a range of temperature estimates. The higher estimates are made on the assumption that the entire world will continue to use more and more fossil fuel per capita. The lower estimates come from best-case scenarios in which environmentally friendly technologies such as fuel cells and solar panels replace much of today’s fossil fuel combustion. After inputting estimates for future greenhouse gas emissions, scientists run the models forward into many possible futures to arrive at the range of estimates provided in the IPCC report. The estimates are being used to predict how rising temperatures will affect both people and natural ecosystems. The severity of environmental change will depend on how much the Earth’s surface warms over the next century.		Global warming will not affect all places on Earth the same way. Climate models predict that warming will be greatest in the Arctic and over land. Models also give a range of temperature predictions based on different emission scenarios. If humans limit greenhouse gas emissions (low growth), then the temperature change over the next century will be smaller than the change predicted if humans do not limit emissions (high growth). (©2007 IPCC WG1 AR-4.)
	Potential Effects of Global Warming Building a Climate Model		The Intergovernmental Panel on Climate Change estimates that Earth will warm between two and six degrees Celsius over the next century. The range in estimate comes from running different emission scenarios through several different global climate models. Scenarios that assume that people will burn more and more fossil fuel provide the estimates in the top end of the temperature range, while scenarios that assume that greenhouse gas emissions will grow slowly give lower temperature predictions. The orange line provides an estimate of what global temperatures would have been if greenhouse gases had stayed at year 2000 levels. (©2007 IPCC WG1 AR-4.)