Focus on damaging effects of climate change
A sticky question scientists are at odds over as areas of planet cope differently with greenhouse gases
HOW much Earth will warm in response to future greenhouse gas emissions may be one of the most fundamental questions in climate science – but it’s also one of the most difficult to answer.
And it’s growing more controversial. In recent years, some scientists have suggested that climate change will be less severe than projections suggest.
But new research is helping to lay these suspicions to rest. A study, just out in the journal Science Advances, joins a growing body of literature that suggests the models are on track after all.
The new study addresses a basic conflict between what the models suggest about future climate change and what we can infer from historical observations alone. Some scientists have suggested that the models may be too sensitive, pointing out that the warming they predict for the future is greater than what we would expect based only on the warming patterns we’ve observed since humans first began emitting greenhouse gases.
The models, for instance, suggest that if the carbon dioxide concentration in the atmosphere were to reach double its preindustrial level, the planet would warm by anywhere from about 1.5°C to 4.5°C.
But the warming patterns we’ve actually observed over the past 200 years or so would suggest that a doubling in carbon dioxide should only lead to about 3°C of warming. The discrepancy has become a major cause for concern among climate scientists, according to the new study’s lead author Cristian Proistosescu, a research associate at the University of Washington who was completing a PhD at Harvard. Even the UN Intergovernmental Panel on Climate Change acknowledged the issue in its most recent report, he said, stating that it could no longer provide a best estimate for the climate’s sensitivity.
The new study helps to reconcile the models with the historical record. It suggests that global warming occurs in different phases or “modes” throughout the planet, some of which happen more quickly than others. Scientists now increasingly believe that certain slow-developing climate processes will amplify warming to a greater extent in the future, putting the models in the right after all. But these processes take time, even up to several hundred years, to really take effect – and because not enough time has passed since the industrial revolution for their signal to really develop, the historical record is what’s actually misleading at the moment.
This conclusion is supported by a growing body of research which suggests that warming estimates made from the historical record alone are “potentially biased low, for reasons we are now just beginning to understand”, said Timothy Andrews, a climate scientist with the Met Office, the UK’s national weather service.
The new study uses a statistical method to separate “fast” and “slow” climate modes in the models. According to Proistosescu, when greenhouse gases are emitted into the atmosphere, a “fast” warming effect begins to take place almost immediately in certain parts of the planet, mainly over the land masses in the Northern Hemisphere. These are the parts of Earth where the most rapid warming has been observed since the industrial revolution.
On the other hand, Proistosescu said, other parts of the planet – the Southern Ocean and the eastern Pacific – respond more slowly, in part because they’re so deep and cold. But as they absorb more heat and finally start to warm up, they may produce a variety of climate feedback effects that actually enhance the global warming that’s already occurring. For instance, changes in ocean temperatures can alter atmospheric patterns around the world.
The models suggest that these warming ocean regions may lead to a decrease in reflective cloud cover in the future, allowing more solar radiation to make it through the atmosphere to Earth’s surface.
While the impact of these processes may be profound, they can also take long periods of time to unfold, Proistosescu said – potentially up to 300 years or so. The researchers found that the models suggest the effects of this slow climate mode only account for about 3% of the human-caused warming we’ve seen so far. But in the future, under a scenario in which atmospheric carbon dioxide concentrations double, it could account for up to half of all warming that occurs.