A reader asks about global warming...

A few days ago this email appeared in our inbox:

I am constantly bringing up the issue of global warming in conversation & have been telling people about the film An Inconvenient Truth - however I keep on getting the same response from people which is - while parts of the earth are getting warmer other parts are getting colder - so the global warming effect is cancelled out.

They never tell me which parts of the world are getting colder - are there parts of the world that are getting colder? If so where? And does this fit into global warming?

This was my reply:

Thanks for getting in touch and well done on your efforts to inform people about global warming. Perhaps the best thing I can do is point you to the notes on this at Wikipedia, which appears pretty authoritative and helpful:

http://en.wikipedia.org/wiki/Global_warming

You'll see that it's now very well established that average global surface temperature has increased significantly since pre-industrial times, almost certainly as a result of human activities (primarily fossil fuel combustion and deforestation).

I'm not aware of parts of the globe that are cooling - there *may* be - but there is no question of any global warming effect being cancelled out. As you'll see at Wikipedia, average global temperature has risen by about 0.6 deg Celsius in the 20th century - over the whole globe that's a huge amount of increased heat energy! It's that heat that drives the weather, storms, etc.

It may be that people have heard about the "global dimming" effect of airborne pollution that reflects back out to space some of the incoming solar energy. This has masked (not cancelled out) some of the global warming that would otherwise have taken place. Because of improved environmental regulations in many countries, airborne pollution (sulphate aerosols from power stations, etc.) is reducing. Good news in terms of better health, but it does mean that the dominant global warming effect will now become even stronger. Up till now, a kind of brake has acted upon global warming. But now the brake is coming off.

See: http://en.wikipedia.org/wiki/Global_dimming

Some important points to conclude:

It's not just about *average* effects such as the global average temperature increasing, sea level rising, etc. The more energy is in the climate system (more heat) - the more extreme phenomena will occur: severe storms, flooding, droughts, etc. Talking about just "warming" gives the impression that things will change gradually, in a linear manner. But the climate system can have positive feedback effects that suddenly accelerate the changes taking place: e.g. the potential melting of the permafrost in western Siberia (evidence is strong that this is already happening), releasing billions of tonnes of methane - a powerful greenhouse gas - into the atmosphere. That would truly have catastrophic effects.

Same goes for other possible "tipping points" (a buzz phrase much loved by politicians and the media but it's a proper scientific/engineering concept too), such as the melting of the Greenland or west Antarctic ice sheets. Yet another, is the possible weakening or even collapse of the so-called thermohaline ocean circulation in the North Atlantic, which includes the Gulf Stream and its extension, the North Atlantic Drift, which moderates the climate of western Europe. In a warming world, the ocean "pump" that drives this ocean circulation (the pump is the sinking of cool, saline water at high latitudes, which then "drags" up warm water from Florida) would falter. The temperature in western Europe would then drop by 5-6 deg Celsius - even as the rest of the world warms - a huge decrease that would have massive consequences for us. In fact there is evidence in a paper published in Nature by colleagues of mine last December that the Gulf Stream has already weakened by around 30%. Is this a longterm trend or part of a natural, decades-long cycle? We don't know yet - we need to keep monitoring what's going on - but there is lots of mounting evidence (e.g. Arctic sea ice cover not recovering every winter) that observed changes match very closely those that are predicted in a warming world by advanced computer models of climate.

best wishes,
David Cromwell

P.S. I haven't seen Al Gore's film but by all accounts it's very good on the climate science.