I hope Jim Hansen is wrong

I really do.

Hansen’s take on the emerging science behind climate sensitivity is worrying. In a recent speech to the AGU conference he stated:

The long-standing “Charney” problem has been solved. If continents are fixed as at present, ice sheets are fixed, vegetation distributions are fixed – global climate sensitivity for doubled CO2 is about 3°C. This Charney sensitivity includes fast feedback processes – water vapor, sea ice, clouds. Models have inherent uncertainties, but comprehensive empirical data for the last ice age implies a sensitivity of about three degrees.

News to some obscure Viscount wing-nut but, nevertheless, likely to be correct.

Hansen continues:

The size of ice sheets for the past 400,000 years is known from sea level data, and greenhouse gas amounts are known for the same period. Taking these as boundary conditions, or forcings, shows that the same Charney fast feedback sensitivity fits the entire period. However, the ice sheets and greenhouse gases are feedbacks on these time scales, driven by small forcings due to slow changes in the Earth’s orbit. In response to these small forcings the Earth is whipsawed through dramatic climate changes. Positive feedbacks reign supreme.

So studies of the past show that small forcings lead to large climate changes. Some especially large:

Yet these climate changes, however staggering they seem to humans, with 400 foot changes of sea level, and New York, Minneapolis and Seattle under ice sheets thicker than our tallest sky-scraper, are just the “little whip saw”. Consider the changes that have occurred on longer time scales, for example, global warming events such as that at the Paleocene-Eocene boundary, driven at least in part by methane hydrate release.

Go back further to the greatest whip-saw of all, “snowball Earth” events in the Proterozoic, and the most recent one, which ushered in the Cambrian period. The Earth froze all the way to the equator, and after greenhouse gases accumulated and some melting began, the planet was whipsawed to hellish hothouse conditions.

Kind of makes you wonder why denialists think that a Mediaeval Warm Period debunks anthropogenic global warming, doesn’t it?

So how do anthropogenic forcings compare to natural forcings such as orbital and solar-output changes?

We live on a planet whose climate is dominated by positive feedbacks, which are capable of taking us to dramatically different conditions. The problem that we face now is that many feedbacks that came into play slowly in the past, driven by slowly changing forcings, will come into play rapidly now, at the pace of our human-made forcings, tempered a few decades by the oceans thermal response time.

Surely we can adapt. Isn’t a little warming and plant fertilizer supposed to be beneficial?

Civilization developed during the past several thousand years in the tranquil Holocene, temperature hardly changing, shorelines practically fixed. Our infrastructure has been built for that planet. Some previous interglacials were warmer than the Holocene, but, with the warming of the past few decades, we are now within about 1°C of the warmest interglacial. If we follow business-as-usual greenhouse gas emissions, the warming this century due to just the fast feedback processes will approach 3°C. But surely additional feedbacks would start to come into play, with dark evergreen forests moving poleward, tundra melting and possibly releasing methane hydrates, ice sheets beginning to shrink. It would be a different planet, with no sea ice in the Arctic, with many species of life driven to extinction, with ice sheet disintegration and rising sea level out of our control, more intense hot dry conditions in spreading subtropical areas such as the western U.S., the Mediterranean, Middle East and parts of Africa. The semi-arid part of the United States, stretching from West Texas through Oklahoma, Kansas, Nebraska and the Dakotas is likely to have more extensive droughts and be less suited for agriculture. As isotherms move poleward, so too will pests and diseases normally associated with low latitudes.

Give me contrarian sneers over Hansen being correct any day.

The trouble is, Hansen seems to know what he's talking about.