The UnAustralian

Thursday, June 12, 2003
More On Global Warming: Part 1

Ken Parish has jumped into the debate on the emission scenarios that are at the heart of the IPCC's projected warming. His post is well worth the read, and can be found here.

For the purposes of this post, I'm going to ignore significant chunks of Ken's post, and just focus on his critique of the assumptions behind the various IPCC scenarios. This is simply because that's were the meat of his post is.

In a nutshell, Ken finds flaws in the assumptions behind all of the four scenario families. Near the start of his post he summarises it as "although the IPCC does indeed present a wide range of scenarios (no less than 40 of them divided into 4 broad "families"), all of them are distorted by a variety of exaggerated assumptions carefully designed to achieve CO2 emission outcomes much higher than can be justified by existing evidence and research." Interestingly enough, he later shows that this statement is false by presenting projections which show that for certain scenario families, CO2 emissions in 2100 are significantly lower than 1990 levels. How these numbers can be said to be higher than what is justifiable is beyond me.

Rather than try and summarise Ken's argument, it's best to quote his summary and recommend that everybody reads his whole post:

Therefore, current research shows that the IPCC's A2 storyline, which assumes a world population of 15 billion by 2100, is patently absurd. It assumes a world population almost twice as high as current research indicates is likely. Thus, while the A1 and B1 scenarios achieve exaggerated CO2 emission outcomes by (at least arguably) exaggerating likely future economic growth, the A2 storyline does so by drastically exaggerating likely population gowth.

The B2 storyline also exaggerates population growth to an extent, in that it assumes a world population of 10 billion by 2100; some 19% higher than current research suggests is most probable. However, that is at least in the ballpark. The A2 scenario is so far outside it as to be laughable. Although there are clearly very large uncertainties involved in projecting future population (just as there are in projecting economic growth), a projection this far in excess of current mid-range estimates is downright dishonest.

It's hard to criticize Ken's disagreement of the A1 and B1 scenarios as, I'm not an economist, and can't make an informed judgment on whether or not the growth rates are too high. However, it may be illustrative to look at China's recent growth. According to World Bank numbers China's gdp grew at around 7 - 9% p/a from 1997 - 2001. It's population however, only grew at about 1% p/a. Given that the highest IPCC scenario family shows per capita growth at 4.9%, it is clear that countries are capable of growing significantly faster than the highest IPCC estimations. Also, given that China is (per capita) very poor by first world standards, it should be possible for China to maintain high growth levels for a sustained period of time (it's growth will start to level as it closes the gap with the developed world). Given China's massive population, it's effect on Asian (and global) growth rates will be massive. Another example is Japan which has managed very high growth rates for a long period of time.

Ken's arguments against the A2 (and to a lesser degree, the B2) scenario families on the basis of their population are highly flawed. He compares their population estimates with UN numbers, and marks them down for being too high. However, what he hasn't done is compare them with the range of UN numbers. At 2050, the UN projects a global population of between 7.5 and 10.6 billion. The mid-range is at 8.9 billion.

By contrast the IPCC scenarios projections involve the following numbers:

A1: 8.7 billion
A2: 11.3 billion
B1: 8.6 billion
B2: 9.4 billion

Therefore, the A1 and B1 scenarios are pretty similar to the UN mid-range numbers, the B2 scenario is intermediate between the mid and high end, meanwhile, the A2 scenario is a bit above the high end UN numbers.

So, why are the A2 (and to a lesser degree the B2) population numbers higher than UN mid-range estimates. The answer is quite simple, population projections are extremely sensitive to birth numbers, and people in poor countries tend to have more kids. These two scenario families, with their low third world growth, lead to large amounts of poverty and hence lots of extra kids.

| 8:00 PM