If the countries of the world reduced their greenhouse gas emissions today enough to keep the world from warming more than 2 degrees Celsius, when would they be able to tell that these efforts had succeeded?
That's the basic question posed in a paper released yesterday in the journal Proceedings of the National Academy of Sciences.
The answer: about 25 to 30 years, at least where global temperatures are concerned. On a regional level, it may take even longer to see the changes, the paper states.
To those who care about curbing the negative impacts of climate change, this suggests there is no time like the present to start curbing emissions.
"The delay in the emergence of the benefits also suggest that we should start sooner rather than later to think seriously about mitigation," said paper author Claudia Tebaldi, a research scientist at Climate Central and a visiting scientist at the National Center for Atmospheric Research.
Tebaldi and co-author Pierre Friedlingstein, of the University of Exeter, analyzed when scientists would be able to detect the difference between a scenario known as RCP 2.6, where greenhouse gas emissions are curbed quickly, versus two other scenarios outlined in the Intergovernmental Panel on Climate Change reports.
Politics of deferred gratification
Under one of the additional scenarios, known as RCP 4.5, humans take longer to reduce greenhouse gas emissions but eventually do so, and under the other, known as RCP 8.5, carbon dioxide concentrations continue to rise through 2100.
Because the Earth's climate has a certain amount of natural variability, and those natural cycles can have warming and cooling effects that last for a couple of decades or even longer, Tebaldi said, it takes time to detect a change.
From a scientific perspective, Tebaldi said it was interesting to try to separate the signal of man-made climate change, and efforts to curb it, from the noise of this variability.
But the work also has a policy impact. The researchers wanted to set realistic expectations for policymakers working on reducing greenhouse gas emissions.
"We were interested in providing a measure of how long people would have to wait until the effect of mitigation becomes tangible, in terms of avoided warming," Tebaldi said.
"People may wonder if these measures have any effect at all, if they are under the naive expectation that mitigation of emission will immediately and straightforwardly produce some benefit. That's not the case."
The case of having a long lead time before seeing benefits is not unique to climate change policy, however.
Cleaner air and water will come more quickly
According to David Doniger, policy director of the Natural Resources Defense Council's climate and clean air program, it took the benefits of the Montreal Protocol, which aimed to reduce emissions of ozone-depleting substances, years to come to fruition.
Curbing greenhouse gases is somewhat akin to a farmer planting a tree, Doniger said. "It won't produce any shade in his lifetime, but he does it for his children."
Of course, there are some near-term advantages to curbing emissions that the paper does not address, noted Kelly Levin, a senior associate at the World Resources Institute and an expert in emissions reductions.
"There are going to be immediate co-benefits that you are going to see immediately," Levin said.
These are the cleaner air and cleaner water that result as a byproduct of curbing carbon emissions. The IPCC has said the reduction in air pollution from curbing greenhouse gas emissions could prevent tens of thousands of deaths in Latin America and Asia.
Earlier this year, NRDC put together an analysis quantifying the benefits of reducing carbon dioxide emissions from U.S. power plants. That report calculates the benefits of saved lives and fewer illnesses from reduced pollution associated with curbing emissions is between $11 billion and $27 billion by 2020.
In fact, Levin said, a lot of developing countries are interested in reducing greenhouse gas emissions and investing in clean development because they get these benefits.
"They're thinking about this in terms of the benefits to development, and that it's going to be a cleaner pathway," she said.
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