One of the biggest challenges in predicting the future climate of Earth is taking account of future human behaviors, which will depend greatly on political and economic factors. Burning coal, for example, contributes both greenhouse gases which warm the climate; dirty coal plants produce sulfate aerosols, which tend to cool it, and black carbon aerosols, which have a warming effect. The unpredictable human factor comes in addition to the remaining uncertainties in our understanding of how various physical effects force the climate. One way to separate those two uncertainties is to examine the impact that human activities have already had on the climate.

Characterizing the accumulated human effect on climate also enables us to predict what happens to future climate if we stop all anthropogenic emissions today. Even though that's an unrealistic situation, it can be informative: if we know what effect our activities up to now have had on the climate, we can also predict what effect the same accumulated human activities will have in the future. This way, we can estimate the changes we are already committed to; thus scientists call this the climate commitment.

A new study published on January 15 in Geophysical Research Letters, done by Kyle Armour and Gerard Roe of the University of Washington in Seattle, refines the calculation of climate commitment by incorporating the effect of aerosols emitted through human activities. Previous studies took account of greenhouse gases only, and these found that the anthropogenic greenhouse gases released so far will cause 0.6º C rise above the pre-industrial global temperature. The warming is projected to stay for the indefinite future, and the temperature is unlikely to return to the preindustrial level even if all anthropogenic emissions are stopped today, and the planet is allowed to return to relatively stable conditions on its own.

The new study adds the effect of anthropogenic aerosol emissions to the framework of the previous studies. Particulate pollutants are known to mask the greenhouse effect and can keep the global temperature cooler. As particulate pollutants are relatively easy to wash out through precipitation, however, their effects are also short lived unless their concentrations are maintained by human activities.

The implication of the particulate pollutants’ short residence time is that the masking of greenhouse effect goes away about 30 years after we cease all emissions. At that point, the temperature suddenly spikes, most likely by about 0.1º C, above the current temperature and 0.9º C above the preindustrial levels. However, uncertainties in the magnitude of aerosol forcing leaves open a range of scenarios, and the rapid temperature rise could be as strong as 0.7º C (1.6º above pre-industrial) in some cases.

In other words, the new study shows that the climate commitment has a transient phase in which most warming comes from the unmasking of greenhouse effects as anthropogenic aerosols are washed out of the atmosphere.

In the long-term, the warming caused by the lack of aerosols is eventually balanced by the leveling off of the greenhouse effect as the greenhouse gases are absorbed by the environment; however, it still leaves the planet 0.6º C warmer than the preindustrial level, so the long-term climate commitment derived in the new study is the same as the previous calculations, which did not include aerosols.

The agreement in the long-term climate commitment between the current and previous studies is not surprising—the new study accounts for greenhouse gases in the same way as the ones before, so their scenarios converge when aerosols go away. However, the new work illuminates the importance of properly taking aerosol forcings into account, as they remain one of the major uncertainties in predicting future climate.

Studies such as the one by Armour and Roe reaffirm the extent of the modifications human activities have performed on the global environment. Anthropogenic forcing of the climate has both positive and negative components, so the new study shows that stabilizing the future climate is not as simple as immediately stopping all emissions today, as the transient warming that occurs as aerosols wash out of the atmosphere could trigger other feedbacks that drive the climate in unpredictable ways.

The high level of human activities on Earth means that changes in the global environment are simply unavoidable. It may be difficult to predict how well-behaved human beings will be in the future, however, so we must learn to actively manage both positive and negative effects in order to keep our planet comfortably livable—of course, the strongest of the forcing comes from the carbon dioxide emissions. The new study helps us understand the future impacts of our current activities.