Forcing Agents Underlying Climate Change: Page 5 of 11
Submitted Testimony: 3. Past Climate Forcings and Climate Change.
The climate forcings that exist today are summarized in Figure 2 (1). The greenhouse gases, on the left, have a positive forcing, which would tend to cause warming. CO2 has the largest forcing, but CH4, when its indirect effect on other gases is included, causes a forcing half as large as that of CO2. CO2 is likely to be increasingly dominant in the future, but the other forcings are not negligible.
Aerosols, in the middle of the figure, are fine particles in the air. Some of these, such as sulfate, which comes from the sulfur released in coal and oil burning, are white, so they scatter sunlight and cause a cooling. Black carbon (soot) is a product of incomplete combustion, especially of diesel fuel and coal. Soot absorbs sunlight and thus warms the planet. Aerosols tend to increase the number of cloud droplets, thus making the clouds brighter and longer-lived. All of the aerosol effects have large uncertainty bars, because our measurements are inadequate and our understanding of aerosol processes is limited.
If we accepted these estimates at face value, despite their large uncertainties, we would conclude that, climate forcing has increased by 1.6 W/m2 since the Industrial Revolution began [the error bars, in some cases subjective, yield an uncertainty in the net forcing of 1 W/m2]. The equilibrium warming from a forcing of 1.6 W/m2 is 1.2-1.3° C. However, because of the ocean's long response time, we would expect a global warming to date of only about 3/4 ° C. An energy imbalance of 0.6 W/m2 remains with that much more energy coming into the planet than going out. This means there is another 1/2 ° C global warming already in the pipeline — it will occur even if atmospheric composition remains fixed at today's values.
The climate forcings are known more precisely for the past 50 years, especially during the past 25 years of satellite measurements. Our best estimates are shown in Figure 3. The history of the tropospheric aerosol forcing, which involves partial cancellation of positive and negative forcings, is uncertain because of the absence of measurements. However, the GHG and stratospheric aerosol forcings, which are large forcings during this period, are known accurately.
When we use these forcings in a global climate model (3) to calculate the climate change (6), the results are consistent with observations (Figure 4). We make five model runs, because of the chaos in the climate system. The red curve is the average of the five runs. The black dots are observations. The Earth's stratosphere cools as a result of ozone depletion and CO2 increase, but it warms after volcanic eruptions. The troposphere and the surface warm because of the predominantly positive forcing by increases of greenhouse gases, in reasonably good agreement with observations
The fourth panel in Figure 4 is important. It shows that the simulated planet has an increasing energy imbalance with space. There is more energy coming into the planet, from the sun, than there is energy going out. The calculated imbalance today is about 0.6 W/m2. This, as mentioned above, implies that there is about 0.5° C additional global warming already in the pipeline, even if the atmospheric composition does not change further. An important confirmation of this energy imbalance has occurred recently with the discovery that the deep ocean is warming. That study (7) shows that the ocean took up heat at an average rate of 0.3 W/m2 during the past 50 years, which is reasonably consistent with the predictions from climate models. Observed global sea ice cover has also decreased as the models predict
There are many sources of uncertainty in the climate simulations and their interpretation. Principal among the uncertainties are climate sensitivity (the Goddard Institute for Space Studies model sensitivity is 3° C for doubled CO2, but actual sensitivity could be as small as 2° C or as large as 4° C for doubled CO2), the climate forcing scenario (aerosol changes are very poorly measured), and the simulated heat storage in the ocean (which depends upon the realism of the ocean circulation and mixing). It is possible to find other combinations of these "parameters" that yield satisfactory agreement with observed climate change. Nevertheless, the observed positive heat storage in the ocean is consistent with and provides some confirmation of the estimated climate forcing of 1.6 ± 1 W/m2. Because these parameters in our model are obtained from first principles and are consistent with our understanding of the real world, we believe that it is meaningful to extend the simulations into the future, as we do in the following section. Such projections will become more reliable and precise in the future if we obtain better measurements and understanding of the climate forcings, more accurate and complete measures of climate change, especially heat storage in the ocean, and as we employ more realistic climate models, especially of ocean circulation