The latest and most definitive report of the international body set up to review the scientific evidence on climate change will be littered with the language of uncertainty. But the overall message is clear enough: the global climate is changing and human activity is largely responsible.
Scientists are a cautious lot when it comes to making statements on things where there is limited empirical evidence. They are even more cautious when making predictions about the future, especially over timespans of many decades or centuries.
This is why the fifth assessment report (AR5) of the UN’s Intergovernmental Panel on Climate Change, to be published next month, will be peppered with phrases, such as “virtually certain”, “very likely” and with “high confidence”. But this uncertainty should not be taken as a reason for political inaction.
There are some things now that are beyond dispute, notably the fact that human activity has increased carbon dioxide concentrations by about a half since the start of the Industrial Revolution – from less than 270 parts per million to nearly 400 ppm, and rising. Secondly, global average temperatures have increased by about 0.8C between 1901 and 2010, and by about 0.5C between 1979 and 2010.
A number of key indicators of global climate change show an accelerating effect. The frozen “cryosphere” for instance of polar ice and mountain glaciers has melted faster over the past few decades compared to the previous half century, and mean sea levels are rising faster now than at any time in the recent past.
Sea levels will be one of the most disturbing and potentially catastrophic of climate-change impacts, given that some 70 million people in Europe alone live near the coastline. It is now commonly agreed that the last IPCC report of 2007 was unduly conservative in its assessment of future sea-level rise, a notoriously difficult metric to predict.
The latest report will point out that it is “virtually certain” that over the 20th Century the mean rate of increase was between 1.4 to 2.0 millimetres per year, and that this has increased to between 2.7 and 3.7mm per year since 1993.
The melting ice alone could add up to 37cm to mean sea level by 2100 and the thermal expansion of the warmer oceans could add about the same again. Even if we stop all carbon dioxide emissions tomorrow, much of this increase is unavoidable because of the basic inertia of the global climate – although the earlier we start curbing greenhouse gases, the easier it will be avoid the worse possible outcomes.
Scientists believe that huge amounts of heat from the atmosphere have been taken up by the deep ocean – equivalent to the power of 150 billion electric kettles – offsetting to some extent the increase in surface heating due to human activity. This could account for the slight “pause” in global surface temperatures seen over the past decade, but if so it will be short lived.
Another indisputable fact is that despite the global recession the amount of greenhouse gases emitted from industrial and domestic sources has continued to increase at an astonishing pace, putting us in the “worse-case” scenario in terms of future climate trends.
The essential point behind all this work is that we are changing the global climate and could end up having to live in a very different and inhospitable world to the one we have always known.