If polar ice caps melt quickly, sea levels will rise much faster than scientists think

James Hansen, director of Nasa’s Goddard Institute for Space Studies, has for long been one of the most prominent figures in climate change research. Ever since the 1980s, he has been an outspoken critic of White House policies on global warming. Recently, he privately circulated a paper on the cost of scientific reticence, a tendency among scientists to be cautious and absolutely sure before making a public statement. This paper is generating interest in the community of climate scientists and will be published soon in Environment Research Letters.

Hansen’s target is the Intergovernmental Panel for Climate Change (IPCC), a body that is known to be cautious in making statements about the future. Hansen acknowledges that IPCC needs to be cautious, and scientific objectivity demands that we speak only about things we know. But his problem is that this cautiousness is stopping it from communicating important things to the public. Specifically, he thinks the IPCC is not communicating to the public and policy makers the threat of a potentially large rise in sea level. IPCC is right to be cautious, he says in the paper, but excessive caution is dangerous as the mechanism of sea level rise has a built-in delay. By the time we are sure, it may be too late.

Hansen is agitated because the IPCC has ignored the effects of melting ice sheets on sea level rise. IPCC has its estimates about sea level rise — between 28 and 42 centimetres by the end of the century. It takes into account factors like thermal expansion — water expands as it warms up beyond four degree centigrade — and some ice melting, but it does not consider fully the effect of melting polar ice caps. This is because there is very little in scientific literature that IPCC can use. We do not understand ice caps well; we do not know how quickly they will melt. Since IPCC uses only things that are understood to science, it did not fully consider polar ice melting in its models and used what we understand well. What is wrong with that approach?

From a scientific point of view, there is nothing wrong. But if ice caps melt quickly, sea levels will rise much faster than we think. So foresight demands that we should be prepared. Hansen thinks there are positive feedbacks in polar ice melting that we cannot afford to ignore. Ice reflects sunlight back into space. If there is less ice, less heat is reflected and this causes more ice to melt. “Ice sheets are the thing when it comes to sea level rise,” says Hansen. The problem is that the effect can be non-linear, which means our correlations between temperature rise and sea level rise may not hold any more after some point. No one knows when we will approach this point. We may have already passed it (the most unlikely scenario), we may pass it in the next few decades, or we may pass it by the end of the century. “I think IPCC should make it clear that ‘business as usual’ scenarios for climate change imply an almost certain sea level disaster,” says Hansen.

Hansen’s view is shared by a few other scientists. While runaway change for the worse is a clear possibility, there are unknowns that could change things the other way, although we can speak about them with even less certainty. One of them is the behaviour of the sun. Our star goes through periods of high activity, alternating with quieter periods. An active sun, like it is now, produces a warm climate on the earth, and quieter periods cause mini ice ages. Such cycles have happened throughout its history, even human history. The last mini ice age occurred in the 18th century. Temperatures across the globe plunged a few degrees. Europe experienced some of the coldest periods in recent times.

There are scientists who think the sun is due for a quiet period, in which case it will counteract the effects of global warming. The planet could stay as it is without warming. The increasing levels of carbon dioxide will act as a blanket and balance the effects of a cooler sun. There won’t be a mini ice age. There won’t be a global warming either. It would be the perfect solution, if only we were certain.

Like in the case of the ice sheets, we understand very little about the sun. We know that it goes through these cycles, some lasting a decade, some lasting a few decades, and some lasting probably centuries. We do know that it will cool marginally for a few decades sometime in the near future. But we do not know precisely when. So no one can issue predictions on this basis. Says Arnab Rai Choudhuri, physics professor at the Indian Institute of Science, Bangalore, “The sun has indeed gone into quiet periods in the past but there is no regularity to this phenomenon. No one can say whether it will happen in 20 years or 200 years.”

If the sun indeed cools down, we will get more time to reduce greenhouse gas levels. But it can act up again and make us worse off. Which is why we would do better to ignore the sun altogether in our plans.