They keep coming out of the woodwork! The fourth sceptical response was not written to me but whispered in my ear. ‘Michael‘ , said the mischievous voice, ‘People cite the retreat of glaciers as evidence of global warming, but glacial retreat began in the nineteenth century.‘ This voice was careful not to say what appeared by the logical sequitor: ‘That therefore it could not have been initiated by anthropogenic climate change.‘ The voice did not say this, but it thought it.
I was well aware of this. There are many dramatic pictures of disappearing Glaciers, and they are a very tangible indicator that ‘something appears to be happening’. But I too had been puzzled by the early onset of Glacial Retreat. At times like this I find a look at the data always helps.
The bar chart shows the number of glaciers observed to retreat (red) or advance (blue) each year from 1845 to 2005. The data cover the whole Earth, but the early data is biased towards Europe. What do the data tell us?
- Glacial retreat in Europe began before 1900.
- World wide, the number of glaciers currently in retreat vastly outnumbers advancing glaciers
- Some Glaciers are still advancing
The position of a glacier stays constant when it is in mass balance: as much ice melts each year as accumulates as snow. On the one hand, we might expect that a warmer planet should have more precipitation and so lead to positive mass balance and hence Glacial Advance. But on the other hand, a warmer planet might be expected to increase summer melting, which might cause Glacial Retreat. So without a detailed knowledge of the exact balance of melting and precipitation, it is difficult to come to a conclusion.
What do the experts say? Well they say lots and lots and lots. Allow me to précis the information for you.
- The overall shrinking of glaciersand ice caps since their maximum in the LittleIce Age in the 14th Century is correlated with the increase in global mean air temperature of about 0.75 °C since the mid 19th century.
- That means very little. It just says that two things have happened at the same time.
- On decadal or regional scales (i.e. locally), changes in snow accumulation may have dominated glacier response in maritime climates. So in the European Alps advances and retreats are driven by changes in winter precipitation (snow ) rather than temperature. Increased precipitation is also seen as the main reason for the glacier advances in the early 18th century and the 1990s in Norway, in the 1990s in New Zealand , and for the 20th century advances and/or thickening of some glaciers in central Karakoram.
- So advances are driven by local or short term increases in precipitation.
- The period in which glaciers were close to steady state or even advancing, which occurred worldwide around the 1970s, might be explained, at least in part by diminished solar radiation due to the increase of atmospheric pollution after the mid 20th century. Recent studies have shown that the atmosphere cleared up again in the mid-1980s, probably as a result of the implementation of industrial filters and the breakdown of industry in the former Soviet Union, which increased the amount of incoming solar radiation and, hence glacier melting.
- Cleaning up our pollution has exposed the full scale of the other changes we have caused. Bring back the smog!
- In addition to climate changes on the global level, altered atmospheric circulation patterns can have an impact on the glacier behaviour of entire mountain ranges. Examples include the mass balance variations of glaciers in the tropical Andes which are strongly influenced by the El Niño-Southern Oscillation.
- Regional climate changes have driven the retreat of some glaciers.
The upshot of all this is that glaciers are exquisitely balanced, and the advance or retreat of a glacier acts as an ultra-sensitive detector of local or regional climate change. The fact that these ‘local’ climate changes are happening ‘all over the planet’ seems to indicate that Global Climate Change – anthropogenically-induced or otherwise – is really happening.
It seems likely that a warming trend is one of the causes of Glacial Retreat, but the trend was well underway before the strong surge in Global Mean temperature began in the 1970′s. Does that prove that Climate Change is not anthropogenically caused? No. It shows that it is difficult to tease out anthropogenically-induced Climate Change from other changes. Nobody said anything would be easy or simple.
Tags: Glacier Retreat