Global Warming Evidence

On July 11th, 2006 Professor Mark Thoma posted an article from the Financial Times entitled 'What do we do now the climate wolf is at the door?'. If "what we put up stays up for 200 to 300 years" and a majority of the CO2 emissions have been in the past 100 years, how will this world look in 2150? Here are some snippets from the article:

The little boy who cried “wolf” was finally proved right and was gobbled up as punishment for his earlier pranks. Malthusians have been crying wolf for a couple of centuries. But in global warming they may well have seen a real one. ... Is global warming a wolf at our door?

The argument that it is starts with the observation that some atmospheric gases generate a greenhouse – or warming – effect. This is an excellent thing since they make the earth’s surface temperature about 30ºC warmer than it would otherwise be. But, as concentrations of greenhouse gases rise, so will the the temperature (... other things ... equal)...

Since the 18th century, there has been a big increase in the output of man-made (anthropogenic) CO2, largely as a by-product of the burning of fossil fuels... Concentrations of CO2 in the atmosphere have risen from about 280 parts per million in the 18th century to 380 parts today. This is ... higher than in the last 420,000 years and possibly the highest in 20m years (though levels have been higher still in the more distant past). The 10 warmest years on record have occurred since 1994, with an average global warming of 0.7ºC since 1900.

Concentrations of CO2 are headed much higher still. Under plausible assumptions, human-caused emissions of greenhouse gases... will tend to rise over the next half century. ... because of the rising use of fossil fuels... [and] further deforestation. ...

If trends of the past half century were to continue, concentrations would reach 550 parts per million by 2050. That level would itself raise global temperatures by between 2.2ºC and 3.6ºC by 2100. The continents might warm by between 2.2ºC and 6.2ºC and the Arctic by between 3.6ºC and 11.4ºC. Such changes could well be associated with extreme events: reversal of the oceanic currents...; and the melting of permafrost and subsequent release of huge quantities of methane. Feedback effects might push temperatures higher than at any time in the last 50m years. The world would be a different place...

I have no intellectual difficulty with this argument, since it is grounded in scientific reasoning. Nevertheless, it raises several further questions. First, how certain are we of the magnitudes of potential warming? Second, how far is the warming itself a “bad thing”? Third, is there any chance that we will, in practice, find a workable way of dealing with it? Finally, what can and should we do about it, while taking into account both the benefits and the costs of any actions?

The answer to the first of these questions is that there remain substantial uncertainties in long-run climate forecasts, as can be seen from the ranges I have given. ... But the forecast direction of change at least seems plausible...

The answer to the second question is trickier... It is, after all, not obvious why a warmer world would be such a bad thing. ... There would certainly be beneficiaries of global warming, perhaps very many of them. But sudden changes impose huge costs of adjustment that would include the disappearance of habitats. Life would survive this... But the adjustment would surely prove disruptive, with an overwhelming probability that the poor would suffer most...

Now we come to the hard questions – what will, can and should be done? The answer to the first is already quite clear: next to nothing. Emissions continue to rise, ... since 1990 aggregate human emissions ... have risen at 1.1 per cent a year. ... Above all, emissions are above the level needed to stabilise concentrations of greenhouse gases (measured in CO2 equivalents) even at 550 parts per million.

For that to happen, emissions in 2050 would need to be no greater than they were in 1990 (which was 12 per cent below 2002 levels) and as much as 50 to 60 per cent below levels forecast for 2050... It is important to note, moreover, that ... even cutting CO2 emissions by 20 per cent below current trends only postpones the date at which we reach 550 parts per million by 15 years. The cuts have to be bigger than this because what we put up stays up for 200 to 300 years.

A betting person who accepts the growing scientific consensus would wager that global warming is going, like the wolf, to gobble us up. But what could and should we do, instead? Are there technological fixes? Is there a policy regime that might be adopted and (unlike Kyoto) make a difference? Do the benefits of action outweigh the likely costs, or should we merely try to adapt? These are the questions to which I will turn in the final column in this series, next week.