"Planet Earth today: imminent peril"

The headline for this post is stolen verbatim from a section headline in a paper on climate change just published in Philosophical Transactions of the Royal Society A. It's yet another depressing read by NASA's Jim Hansen and five co-authors from the University of California, Santa Barbara and the Doherty Earth Observatory at Columbia University. Hansen has been trying to get our attention for some 20 years now with a series of papers laced with alarmist language, but in an email to the Independent, says "this one probably does the best job of making clear that the Earth is getting perilously close to climate changes that could run out of our control."

The Royal Society has generously made the new paper freely available online in HTML and PDF. Whether or not it constitutes the "best" summary of just how bad things really are, it is a great summary of climate science basics, and I recommend everyone at least try to read the whole thing (26 pages, excluding endnotes, but including lots of graphs).

For those short on time, however, here are the highlights. First there's a wonderful explanation of why we shouldn't worry about the fact that sometimes it gets warmer before the greenhouse gases start building up quickly:

The temperature change appears to usually lead the gas changes by typically several hundred years.... This suggests that warming climate causes a net release of these GHGs by the ocean, soils and biosphere. GHGs are thus a powerful amplifier of climate change, comparable to the surface albedo feedback, as quantified below. The GHGs, because they change almost simultaneously with the climate, are a major 'cause' of glacial-to-interglacial climate change, as shown below, even if, as seems likely, they slightly lag the climate change and thus are not the initial instigator of change.

Now, to the scary parts, including several explicit criticisms of the recent predictions of sea-level rise from the Intergovernmental Panel on Climate Change, which played down the threat posted by melting ice packs on Greenland and West Antarctica (bold-faced emphases are mine):

Global warming of approximately 3°C is predicted by practically all climate models for 'business-as-usual' (BAU) growth of GHGs. Yet [the] IPCC (2001, 2007) foresees twenty-first century sea-level rise of only a fraction of a metre with BAU global warming. Their analysis assumes an inertia for ice sheets that, we argue, is incompatible with palaeoclimate data and inconsistent with observations of current ice sheet behaviour.

BAU global warming (approx. 3°C) would be magnified on the ice sheets, based on general high-latitude amplifications found in palaeo records and in climate models, as well as local ice sheet warming due to albedo flip. As a result, large portions of West Antarctica and Greenland would be bathed in melt water. Already areas of summer melt have increased rapidly on Greenland (Steffen et al. 2004), the melt season is beginning earlier and lasting longer, and summer melt is being observed on parts of West Antarctica.

...

The high fast-feedback climate sensitivity (approx. 3°C for doubled CO2) implies that moderate additional positive feedback can produce large climate change, because climate 'gain' is already not far from unity (Hansen et al. 1984). Thick ice sheets provide not only a positive feedback, but also the potential for cataclysmic collapse, and thus an explanation for the asymmetry of the ice ages. The albedo flip property of ice/water provides a trigger mechanism. If the trigger mechanism is engaged long enough, multiple dynamical feedbacks will cause ice sheet collapse (Hansen 2005). We argue that the required persistence for this trigger mechanism is at most a century, probably less.

...

Despite these early warnings about likely future nonlinear rapid response, IPCC continues, at least implicitly, to assume a linear response to BAU forcings. Yet BAU forcings exceed by far any forcings in recent palaeoclimate history. Part of the explanation for the inconsistency between palaeoclimate data and IPCC projections lies in the fact that existing ice sheet models are missing realistic (if any) representation of the physics of ice streams and icequakes, processes that are needed to obtain realistic nonlinear behaviour. In the absence of realistic models, it is better to rely on information from the Earth's history.

That history reveals large changes of sea level on century and shorter time-scales.... We infer that it would be not only dangerous, but also foolhardy to follow a BAU path for future GHG emissions.
...
Given the estimated size of fossil fuel reservoirs (figure 6b), the chief implication is that we, humanity, cannot release to the atmosphere all, or even most, fossil fuel CO2. To do so would guarantee dramatic climate change, yielding a different planet than the one on which civilization developed and for which extensive physical infrastructure has been built.
...
The imminent peril is initiation of dynamical and thermodynamical processes on the West Antarctic and Greenland ice sheets that produce a situation out of humanity's control, such that devastating sea-level rise will inevitably occur. Climate forcing of this century under BAU would dwarf natural forcings of the past million years, indeed it would probably exceed climate forcing of the middle Pliocene, when the planet was not more than 2-3°C warmer and sea level 25 +/-10m higher (Dowsett et al. 1994). The climate sensitivities we have inferred from palaeoclimate data ensure that a BAU GHG emission scenario would produce global warming of several degrees Celsius this century, with amplification at high latitudes.

Such warming would assuredly activate the albedo-flip trigger mechanism over large portions of these ice sheets. In combination with warming of the nearby ocean and atmosphere, the increased surface melt would bring into play multiple positive feedbacks leading to eventual nonlinear ice sheet disintegration, as discussed by Hansen (2005). It is difficult to predict time of collapse in such a nonlinear problem, but we find no evidence of millennial lags between forcing and ice sheet response in palaeoclimate data. An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway. With GHGs continuing to increase, the planetary energy imbalance provides ample energy to melt ice corresponding to several metres of sea level per century (Hansen et al. 2005b).

...

Our concern that BAU GHG scenarios would cause large sea-level rise this century (Hansen 2005) differs from estimates of IPCC (2001, 2007), which foresees little or no contribution to twenty-first century sea-level rise from Greenland and Antarctica. However, the IPCC analyses and projections do not well account for the nonlinear physics of wet ice sheet disintegration, ice streams and eroding ice shelves, nor are they consistent with the palaeoclimate evidence we have presented for the absence of discernable lag between ice sheet forcing and sea-level rise.

...

Present knowledge does not permit accurate specification of the dangerous level of human-made GHGs. However, it is much lower than has commonly been assumed. If we have not already passed the dangerous level, the energy infrastructure in place ensures that we will pass it within several decades.

Hansen et al. wrap up with a bit of a scientific non-sequitur, by delving in their preferred choices for mitigation policy. Not that that's a bad thing, but there's lots of room for argument in their choices.

We conclude that a feasible strategy for planetary rescue almost surely requires a means of extracting GHGs from the air. Development of CO2 capture at power plants, with below-ground CO2 sequestration, may be a critical element. Injection of the CO2 well beneath the ocean floor assures its stability (House et al. 2006). If the power plant fuel is derived from biomass, such as cellulosic fibres grown without excessive fertilization that produces N2O or other offsetting GHG emissions, it will provide continuing drawdown of atmospheric CO2.