Uncertainty about global warming strengthens the need to reduce greenhouse gas emissions

Posted August 11th, 2010 in Climate Change

By Mark Lowey
          The world’s top climate scientists say there is a lot of uncertainty about how much the planet will warm because of rising greenhouse gases in the atmosphere, an international study has found.
           But it’s this very uncertainty that actually bolsters the argument to reduce greenhouse gas emissions, to lower the risk of catastrophic global warming, says a University of Calgary scientist and one of four authors of the study.
           The findings suggest that the United Nations Intergovernmental Panel on Climate Change – the world’s leading scientific body for assessing climate change and its impacts – probably underestimates the uncertainty in predictions about global warming, says David Keith, director of the U of C’s Institute for Sustainable Energy, Environment and Economy’s energy and environmental systems group. 
          “The study also suggests that uncertainties about climate change will not be resolved for decades,” says Keith, a professor in the Schulich School of Engineering. “But uncertainty is not justification for inaction, any more than uncertainty about the probability of car crashes is a reason to forego seatbelts.” 
          “Just as with car crashes or industrial accidents, it’s the low-probability but high-consequence events that drive the overall risk assessment,” Keith notes. “The fact that we may be underestimating these risks strengthens the case for action to reduce carbon dioxide emissions.”
          The study, which included the U of C and University of Victoria in Canada, Carnegie Mellon University in the U.S. and the University of Oxford in the U.K., involved in-depth, face-to-face interviews with 14 leading climate scientists in Canada, the U.S. and the U.K.
           The peer-reviewed research was published in the Proceedings of the National Academy of Sciences – one of the world’s top scientific journals. 
           Researchers interviewed the climate scientists using a method called “expert elicitation,” designed to explore quantitative estimates of the probability of climate change.
           During the interviews, the scientists were asked to consider scenarios of low, medium, and high increases of CO2 in the atmosphere to the year 2200, and to offer their expert judgments about the increase in Earth’s average temperature under each scenario. 
           They also were asked to specify the probability that each of the scenarios would force the planet’s climate system past a “tipping point.” For the high-CO2 increase scenario, 13 of the 14 climate scientists responded that the probability was greater than 50 per cent, and 10 scientists said the probability was 75 per cent or more.
           Such tipping points in the climate system could lead to the loss of a large portion of the Greenland and West Antarctic ice sheets, or the shutdown of currents in the Atlantic Ocean that help moderate climate on a continental scale.
          “Our study of the top experts on the planet found a greater uncertainty on both ends of plausible future scenarios – warming that is relatively benign versus warming that’s likely to be catastrophic,” Keith says.
          “It is the greater uncertainty about whether global warming is likely to be catastrophic that should concern us all. Quantifying this uncertainty is crucial in shaping policy on how much and how fast we need to cut emissions.” 
            The climate scientists also were asked to identify and rank the physical processes on the planet that influence the uncertainty about climate change.
           They said that the No. 1 process causing uncertainty is the role clouds will play in the future in either cooling or heating up the planet.
           The tops of clouds reflect solar radiation back into space and so have a cooling effect. But clouds can also act as an insulating layer, reflecting heat back toward the Earth’s surface.
            Scientists aren’t sure whether the net effect of future changes in cloud cover in a warmer world will moderate global warming or worsen it.
            The No. 2 process the climate scientists identified as causing uncertainty is the “ice albedo feedback” – the solar radiation the Earth’s ice sheets reflect back to space which has a cooling effect, and the amount of warming expected if this ice cover melts.
           Keith says that if one of government’s major goals is to reduce the uncertainty over climate change, then the study suggests that most of the available research funding should go toward investigating these two processes.
           However, even another two decades of research is not expected to resolve all the uncertainties in such a “hard problem” of predicting future climate change, Keith says, “because the uncertainties are deep and not easily removed.”           

Full study is available at:

 http://www.pnas.org/content/107/28/12451.full.pdf+html

EnviroLine