Projections of Future Climate
What will the climate be like in twenty, fifty, or one hundred years? It turns out there is no definite answer to this question, in large part because the decisions we make today, such as a continued reliance upon fossil fuels, will impact the climate of tomorrow. What we can say, using climate models, is how the climate will change if we follow a particular scenario. The Intergovernmental Panel on Climate Change (IPCC)1 has developed various scenarios that characterize possible future worlds based on the amount of greenhouse gases and aerosols emitted over the next hundred years. These scenarios are called "storylines," and each storyline describes how the population grows, the type of global economies present, and the types of energy in use.2 Economists and social scientists work to develop realistic scenarios that encompass a range of possible future worlds. At one end are the high-emission scenarios, which describe a world that promotes and values very rapid economic growth in a material-based economy that relies heavily on fossil fuels. At the other end are the low-emission scenarios, where the world has moved toward more service and information economies and the adoption of clean and efficient-energy technologies. From both of these examples, the amount of greenhouse gases in the atmosphere can be calculated and then fed into a climate model to predict how the climate will change under these conditions. In this manner, we can estimate the range of possible climates expected under these scenarios. Predictions from more than twenty climate-model simulations using both low- and high-emission scenarios from the most recent IPCC report 3 are shown in Figure 1. In the low-emission scenario, temperatures will rise by about 3 degrees F (1.8 degrees C) by the end of the twenty-first century, whereas in the high-emission scenario, temperatures may be up to 6.5 degrees F (3.6degrees C) warmer than they are today.
Although models also vary in their predictions based on different model formulations and assumptions, all models and scenarios predict steady warming throughout the twenty-first century. A couple of points should be highlighted. First, remember that the Earth is quite sensitive to seemingly small temperature changes. If the climate shift from the ice age of 20,000 years ago to current conditions was marked by an 8 degree F (4.4 degree C) increase in surface air temperature, then a further 8 degree F (4.4 degree C) increase in one hundred years, as seen in the high emission scenarios, might also cause some very noticeable changes. The other point is that the low-emission scenario is essentially the goal of the Kyoto Protocol, an international agreement aiming to stabilize greenhouse-gas concentrations at a level that prevents dangerous climate change. So even in this case where significant emission reductions are made, the Earth will continue to warm over the next century. It should be noted that even if greenhouse-gas concentrations were to stay constant at year 2000 levels, the accumulated heat in the oceans would still cause increases in temperature of about 1 degree F (0.5 C) over the next hundred years.3 Although predictions of the future are typically complicated, the message here is that our climate will continue to warm, and, if left unchecked, the warming will push the Earth's temperature to a level that will more than likely be dangerous to natural systems on the planet.4
The scenarios are like paths, and presently we have the choice of which path to take. As you can see, the paths all start relatively close together, but by around 2030 they start to diverge. If society wants to avoid the largest increases in temperature associated with the high-emission scenarios, the challenge will be to move onto one of the lower emission paths within the next decade.
Figure 1. Climate model projections for temperature in the twenty-first century for different scenarios. Solid lines are climate model global averages of surface warming for the high-emission scenario (A2), middle scenario (A1B), and low-emission scenario (B1), shown as continuations of the twentieth century simulations. Shading denotes the uncertainty range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values.5
1. The IPCC was established in 1988 through the United Nations Environment Program and the World Meteorological Organization to assess the risk of climate change caused by human activities. This international body produces reports every five to seven years that are written by climate change experts from around the world. The IPCC Climate Change: Fourth Assessment Report was released in 2007 and can be found at http://www.ipcc.ch/ipccreports/assessments-reports.htm, accessed May 8, 2008.
2. The development of emission scenarios and their storylines is an interesting and somewhat complicated exercise, as described in the IPCC Special Report on Emission Scenarios found at http://www.ipcc.ch/ipccreports/sres/emission/index.htm, accessed May 8, 2008.
3. IPCC, Solomon, et al. (2007).
5. Figure 5 comes from the summary for policy makers from the 2007 IPCC report Solomon, et al. (2007).