by Professor Martin Siegert, Co-director, Grantham Institute
See a headline on TV about climate change and it will undoubtedly be accompanied by, among other images, a glacier suddenly calving a colossal iceberg into the sea, or a polar bear clinging helplessly to an ever-diminishing ice floe.
Glaciology, the study of snow and ice in all its forms, gets into every living room, hammering in the message that when the climate warms up, ice melts.
Of course this is perfectly true. At atmospheric pressure, ice at zero degrees centigrade will melt provided there is energy available for the ice to change its state to water. Quite a lot of energy is needed in fact, ~334 joules per gram, which is more than a hundred times the energy needed to raise the temperature of the same gram of ice by 1 degree.
As we have a lot of ice on our planet, especially at the poles, global warming leads to uptake of heat in the ice. As its temperature rises, the ice melts, the resulting water runs off, and, ultimately, sea level rises.
A complex picture, but a simple conclusion
And that’s basically the nub of the issue. Except, as always, the details are important and they reveal the situation to be far more complex. At a recent science meeting ahead of the COP21 climate summit, I convened a session on sea-level change and ice sheets, with a number of leading international scientists, to discuss what we know about sea level change and what we remain ignorant of. We heard how models don’t match measurements, meaning we need better models, that data are insufficient to understand processes, meaning we need more data, and that coupled together, restrictions in models and data mean we have uncertainty on how the sea will rise.
Yet, our observations and models tell us that it is virtually certain sea level will rise in a world warmer than today’s. This is not solely due to melting ice; most of the sea level change measured since the 1800s comes from thermal expansion of the oceans as they receive heat from the atmosphere. This process will continue under further warming of course.
Locked in changes
While the data and models may need improvement, they are certainly good enough for us to understand that the world’s ice sheets, by far the greatest stores of freshwater, are beginning to react to this warming too. Greenland and Antarctica hold enough ice that, if melted, would raise the level of the sea by 7 and 57 m, respectively. Such a change hasn’t happened since the end of the Ice Age, when sea level rose by 120 m over 10,000 years, flooding the land between what is the now the UK and continental Europe (and many other places).
Time to buy a home up a big hill then? Well, no rush on that, at least for most of us in the UK. Even during melting at the end of the ice age, sea level rose by only on average 1.2 m every 100 years or so. We’re not at that level (yet) at around 30 cm per century. But, the rate of change is increasing, and by the end of this century we may see similar rates.
The issue with ice sheets and sea level isn’t necessarily that low lying regions will suddenly become inundated, though low-lying coastal regions are increasingly at risk of rising seas, it is that any changes from the ice are likely to occur over many centuries. In other words, ice sheets take a long time to react to climate change. The consequences of global warming on sea level is therefore ‘locked in’ to our long-term future. The consequences of our actions, or inactions, on tackling global warming induced sea-level change now will be felt by many generations to come.
Our responsibility to act
For Christmas, my wife gave me a copy of Thomas Paine’s ‘Rights of Man’ (first published 1791), which I started reading several weeks ago. While I have to say I’m finding it quite a challenge (18th century philosophy being what it is), my understanding is that Paine was clear in his opinion that we have no right take actions now that will adversely affect future generations. Specifically, he considers that each child born today has the same natural rights as those born previously or in the future.
It is interesting to consider Paine’s philosophy in the context of manmade sea-level change. Clearly the actions we are having on the climate of our planet, if we do nothing to reverse it, will mean future generations will likely inherit a world different to ours, and be faced with challenges to human civilisation that no previous generations have needed to consider. Hence, any inaction on climate change today could be considered, under Paine’s proposition, to be contrary to the future natural rights of mankind.
Facing the facts
The long-term consequences of sea level change from melting ice are huge. Models inform us that for every 1 C of global warming sea level will eventually (after several centuries) be raised by between 1 and 3 m. Hence, even if we limit climate warming to 2C, our future world may have to deal with an eventual sea level up to 6 m higher than now. For that to happen, the bulk of Greenland and West Antarctica would have melted. In other words, we have already failed our future generations. If climate warms by more than 2C the sea level consequences become far greater. For example, if climate warms by 4C, Greenland and West Antarctica will eventually be completely melted.
While the details are debated, the fact is that we know this problem exists today and will need to be dealt with tomorrow. We certainly have no right to do nothing and just let this happen. Our future generations, and the polar bears, depend on us.
Some say that the COP21 climate summit in Paris later this year, and the resolutions stemming from it, will be our last chance to contain climate warming to 2C. We must embrace this moment and accept the changes necessary. It is simply the right thing to do.
Find out more about how and why sea level changes in our new briefing note by Martin Siegert.
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