For all of its apparent complexity, the threat of dangerous anthropogenic global warming (DAGW) formulated at the Rio Earth Summit in 1992 is based on a very simple assumption:
When X = 560, Y = ECS
X = atmospheric concentration of CO2e in parts-per-million
Y = the increase in temperature since pre-1880, in °C
ECS (Equilibrium Climate Sensitivity) = 1°C plus the ultimate net effect of feedbacks
X is taken from the Mauna Loa observatory and Y is provided by five published temperature series, neither being deeply controversial. The sole debatable element is ECS, the assessment of which is described in Wikipedia:
“A committee on anthropogenic global warming convened in 1979 by the National Academy of Sciences and chaired by Jule Charney estimated climate sensitivity to be 3 °C, plus or minus 1.5 °C. Only two sets of models were available: one, due to Syukuro Manabe, exhibited a climate sensitivity of 2 °C, the other, due to James E. Hansen, exhibited a climate sensitivity of 4 °C. According to Manabe, Charney chose 0.5 °C as a not-unreasonable margin of error, subtracted it from Manabe’s number and added it to Hansen’s. Thus was born the 1.5 °C to 4.5 °C range of likely climate sensitivity that has appeared in every greenhouse assessment since…”
After 30 years and billions of dollars researchers now appear to be ready to adjust Charney’s 3 °C best estimate down to 2.5 °C or perhaps less. This is hugely significant for two reasons:
(i) The warming ceiling fixed at Copenhagen and adopted by the following three UN conferences is 2 °C — so this adjustment cuts the feared excess in half.
(ii) Warming up to 2 °C is expected to have net beneficial effects.
As this graph shows, 2.5 °C is barely off the cusp:
It’s pretty clear that the accumulated effect over the whole period until the 2.5 °C level is reached will be a positive experience. And there is a lot of recent research suggesting that the ECS is actually more likely to be 2 °C or less.
Now let’s consider timing. An influential article in a recent Economist claims the ECS is the wrong metric for policymakers to use because feedbacks will keep happening for hundreds of years. It says:
“For that [policy], a more useful measure is the transient climate response (TCR), the temperature you reach after doubling CO₂ gradually over 70 years. Unlike the equilibrium response, the transient one can be observed directly; there is much less controversy about it. Most estimates put the TCR at about 1.5 °C, with a range of 1–2 °C. Isaac Held, of America’s National Oceanic and Atmospheric Administration, recently calculated his “personal best estimate” for the TCR: 1.4 °C, reflecting the new estimates for aerosols and natural variability.”
We’ve already experienced global warming of 0.8 °C since pre-1880 (IPCC calls it “pre-industrial times”) and have found that minor change to be very beneficial. If the TCR is 1.4 °C, then we can expect the remaining 0.6 °C to occur over the next 70 years — i.e., about the same rate of change that we have enjoyed in the past.
So, where is the problem?
These new figures are not necessarily good news for New Zealand. NIWA expects we’ll warm about one-third less than the world in general, so our TCR 1880–2080 would average about 0.9 °C. But NIWA thinks we’ve already had that much, so we can’t count on any more change (unless we continue to depart from global averages). This is unhelpful because our competitors will have longer growing seasons and better yields while we stay static.
Better news might lie in the possibility that AGW impacts are dwarfed by natural variances, as seems to have been the case during the standstill of the last 15+ years. In that case, only God knows what the future climate will be — either globally or in New Zealand.