There are really three big questions here: 1) How much warming is likely to result from a given scenario of human-caused increases in greenhouse gas concentrations? 2) What will that do to local and regional climates? 3) What will be the actual amounts of greenhouse gases added to the atmosphere in the future?
The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) estimates that global average temperature will rise by between 1.4° to 5.8°C by the year 2100. This rather wide range of uncertainty results primarily from the fact that it is difficult to forecast future emissions, and also from the fact that the ultimate warming will depend on the size and direction of many feedback processes in the climate system that cannot be precisely estimated. Changes in atmospheric water vapor and cloud formation are two of the most important processes in this regard. The warming from increased CO2 will be strongly amplified by associated increases in atmospheric water vapor, while changes in the extent of cloud cover and the characteristics of clouds may either enhance or diminish the initial warming. Accounting for the range of uncertainty in these feedbacks results in a range of possible changes in global average temperatures for any given change in CO2, and in the other greenhouse gases.
Future greenhouse gas emissions are the real wild card because they depend on how fast the world economy grows, how fast world population increases, how quickly our energy technology evolves, and how much our land uses change. Most importantly, greenhouse gas emissions will depend on the policies we put in place to reduce the amount of climate change that will eventually occur.
Figure 1 presents a range of possible future paths for CO2 emissions along with two different estimates of the resulting changes in the atmospheric concentrations of CO2.1 One of the important things to note is that atmospheric CO2 concentrations will be higher in the year 2100 than they are now, even in the scenarios in which emission rates eventually decline significantly relative to present rates. This indicates that some climate change will be inevitable. In fact, even if atmospheric CO2 concentrations could be held fixed at today’s levels, global mean temperature and sea levels would continue to rise for several centuries due to the thermal inertia of the oceans. This is commonly called “committed global warming,” which Wigley (2005) estimates could range from 0.2 to more than 1°C. In addition, recall that CO2 has a long atmospheric lifetime, and that emissions cannot be avoided completely – even under the most optimistic assumptions about future innovations in energy technology.
Figure 1. A possible range of carbon dioxide emissions and the resulting atmospheric changes. Source: IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the IPCC. Cambridge, UK: Cambridge University Press. Figure 3.12, page 222.
1The SRES emissions scenarios pictured here were developed as part of the IPCC 2001 assessment process. They represent a wide range of possible futures, as follows:
A1Fl = rapid economic growth, continued reliance on fossil fuels, converging world living standards, world population peaking in mid century and declining thereafter.
A1T = Same as above except with increasing reliance on new technologies using renewable energy rather than fossil fuels
A1B = Same as above except with a balance of fossil and non-fossil fuel sources
A2 = regionally divergent economic growth, continuing population growth, slower and more fragmented technological change
B2 = emphasis on local solutions to economic, social and environmental sustainability, intermediate technological change, economic growth and population growth
B1 = population as in A1, rapid change toward service and information economy, emphasis on clean, highly resource-efficient technologies.
The other important thing to notice is that there are huge differences in projected CO2 concentrations at the end of the century, depending on the development path followed by the world economy and future population growth. Also note that there is some uncertainty about the eventual CO2 concentrations that would result from any given emission scenario – arising largely from our incomplete understanding of possible changes in the uptake and release of carbon by biological processes on both land surfaces and in the ocean.
Scientific understanding of the sensitivity of the climate system to projected changes in the concentrations of CO2 and other trace gases is also imperfect. Different climate models will produce different projected temperature changes because they incorporate different estimates of the parameters that describe the behavior of the climate system. The range of temperature changes projected by the IPCC reflects the combined effects of all of these sources of uncertainty. Figure 2 compares the range of IPCC temperature projections over the coming century with a range of estimated records of Northern Hemisphere average temperature changes over the past 1000 years. The shading represents the range of uncertainty in both the projections and the record of past variation.
Figure 2. Many studies using different proxy records produce a range of reconstructed global temperature records for the past 1000 years. The range is roughly bounded by the low Moberg (2005) reconstruction and the high Mann and Jones (2003) reconstruction. Also shown is a climate model simulation of past climate based on geologic records of volcanic activity, solar variability, and estimated changes in greenhouse gas and aerosol concentrations. These are compared to the global instrumental record starting in the mid-nineteenth century and to projected temperature changes as of 2100 under three IPCC scenarios: A2 (pessimistic) A1B (balanced growth of fossil and non-fossil fuel use) and B1 (optimistic). (courtesy of Caspar Ammann, Climate and Global Dynamics Division, NCAR).
Of course, global average temperature is a very crude metric of climate change. Nobody lives at the global average. What we really care about is what will happen to climate at particular places, and temperature is only one of several important variables. Water supplies, for example, will be affected by changes in temperature, precipitation (including changes in timing and intensity), insolation, humidity, and wind speed, among other factors. In addition, many human and natural systems are likely to be sensitive to changes in extremes (e.g., the frequency and severity of floods and droughts). Unfortunately, the details of climate changes at particular places and times in the future cannot be reliably projected at this time – even if we could reliably project the change in global average temperature. We will discuss this lack of certainty, and its implications for water utility planning below. Here, it is important to emphasize that while the details of local climate changes cannot be projected with high accuracy, we are beginning to accumulate some evidence on the likely characteristics of climate changes on gross regional scales. This body of evidence is sufficient to allow utilities to explore the implications of a range of potential local climate changes that are consistent with projections of global warming.