What will the world look like after 100 years?

Scenarios are not science

December 21, 2009

Pity the politician in 2010: climate change policies pose an unknown but potentially strong temptation to cross party lines — a bit like abortion brought out single-issue voters a few decades ago.

Some political leaders have a messianic urge to save the planet; others have an ideological aversion to intrusive state controls. A few (perhaps) have studied the science in depth, and all have glanced regularly at fickle opinion surveys. But most are stuck with the muddle in the middle, anxious to do whatever will deliver the best outcomes for the country and their constituents.

Many would begin with the risk-averse approach …”we have to rely on the relevant experts in dealing with highly complex issues. Our official advisers tell us there is a significant risk that human-induced emissions of greenhouse gases are contributing to the recent global warming trend.”

Obvious policy implications of this ‘luke-warm’ stance are solid efforts to improve energy efficiency and to encourage promising new technology — perhaps low-emission fuels. A key consideration for any such programmes is that they are likely to deliver net benefits in any event — even if the warming stops or the causation becomes suspect.

To this point, there is no warrant for suppressing economic growth or engineering revolutions in lifestyle through heavy-handed policy mechanisms such as the CPRS (Carbon Pollution Reduction Scheme — the Australian version of the NZ ETS). That could be justified only if our luke-warmist is converted into a full-fledged alarmist by additional advice that “dangerous” planet-threatening warming is set to occur quite soon — some time in the 21st Century.

This step-up in policy risk needs to be accompanied by a comparable step-up in proof. The politico would (or should) now be insisting upon some positive empirical evidence tying fossil fuel use to the forecast catastrophe. But domestic officials will likely deprecate their powers to predict the future at all, let alone the dates and severity of tipping points, irreversible effects and “surprises”. Only the general circulation models (GCMs) orchestrated by the UN’s IPCC try to deal with such imponderables.

The IPCC itself denies any ability to foretell the future. The GCMs are unknowable black boxes which are programmed to convert a given quantity of emissions into a temperature. The all-important question is the expectable growth rate of emissions over the 21st Century.

In 1998, accepting that scientists couldn’t contribute much to this discussion, the IPCC harnessed a group of economists, statisticians, futurists, demographers, sociologists et al. to establish a picture of how the world might develop over the next century. This group eventually brought out a detailed book, the Special Report on Emissions Scenarios (SRES) of 40 diverse story-lines, any of which might conceivably capture the emissions profile of the 21st Century.

The Scenarios are divided into four families, imaginatively styled A1, B1, A2 and B2, and the emissions in year 2100 cover a huge range from 450 ppm to 1250 ppm, with resulting global temperature projections ranging from 1.1°C to 6.1°C.

A 500% margin of error is scarcely a comfortable starting point for policy-making involving (as the Wall Street Journal put it) “vast reordering of human behaviour at almost unimaginable financial cost”. And, obviously, a politician need not contemplate draconian policy responses at all unless he thinks the Scenarios producing the upper two-thirds of this range are probable.

Emissions growth is driven by global population, economic output, energy intensity and fuel choice. Surprisingly few legislatures around the world have used their enquiry powers to monitor these drivers. But in 2005, an all-party Select Committee of the UK House of Lords conducted a public enquiry into “The Economics of Climate Change”. Its unanimous report (which probably offers the best available guide to the SRES for politicians anywhere) made some very important findings, including:

“It seems wrong to attach equal credibility to the scenarios in general — and we believe the IPCC is now working on this issue.” [Note: there was no change.]

We can make a start by rejecting the A2 family, which assumes that world population will grow to 15 billion, and rising, by the end of the century. Nobody believes this. A number of other UN agencies have contracted specialist demographic studies which agree that we are heading for a global peak of 9 billion around 2050, reducing to 7 billion by 2100.

The B2 family is not quite so extreme, but relies on a world population of 10.5 billion by 2100 — 50% above the official estimates. I think we can put this one aside too.

Both A1 and B1 have reasonable population estimates, but A1 assumes faster economic growth, greater convergence between rich and poor countries, lower technology take-up and higher emissions intensity. Which of these should be preferred?

Well, the House of Lords had some definite findings:

“The high emissions scenarios contained some questionable assumptions and outcomes.”

“The Treasury believes average [global] growth of 3% pa over the next 100 years is unlikely.”

“The assumptions made by IPCC about the rate of convergence in per capita incomes should embody less optimistic assumptions.”

So that seems clear. Jettison A1.

That leaves only B1, which offers a temperature increase as low as 1.1°C, with the “most probable” temperature in 2100 comfortably below the 2°C level which is the threshold declared for the Copenhagen conference.

The unanimous Select Committee report went even further, casting doubt on whether temperature results for the scenarios should be relied upon by serious policymakers at all:

“We received a significant amount of evidence on the realism of the IPCC emissions scenarios:

“They may not be consistent with trends over the past 25 years. Total emissions are indeed increasing, but the rates of increase have slowed significantly, as has the carbon-intensity of the world economy.

“They are not capturing recent experience in their short term projections.

“There is an urgent need for a wholesale reappraisal of the emissions scenario exercise.”

Four years have passed since the “urgent” reappraisal was recommended and it’s been ten years since the Scenarios were invented. Energy intensity and female fertility have fallen further, oil and gas prices have rocketed and there has been a global recession. While politicians may not feel comfortable taking a position on “the science”, they have no excuse for either clinging to agnosticism or relying on faceless futurists in issues of “the economics”.

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