The Earth today stands in imminent peril

...and nothing short of a planetary rescue will save it from the environmental cataclysm of dangerous climate change. Those are not the words of eco-warriors but the considered opinion of a group of eminent scientists writing in a peer-reviewed scientific journal.

- UK newspaper The Independent

The peer-reviewed paper published in July 2007 by NASA's James Hansen et al in the Philosophical Transactions of The Royal Society concludes:

The gravest threat we foresee starts with surface melt on West Antarctica and interaction among positive feedbacks leading to catastrophic ice loss...

West Antarctica seems to be moving into a mode of significant mass loss (Thomas et al. 2004). ...We find it implausible that BAU [business as usual] scenarios, with climate forcing and global warming exceeding those of the Pliocene, would permit a West Antarctic ice sheet of present size to survive even for a century.

Our concern that BAU GHG [greenhouse gas] 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.

The best chance for averting ice sheet disintegration seems to be intense simultaneous efforts to reduce both CO₂ emissions and non-CO₂ climate forcings. As mentioned above, there are multiple benefits from such actions. However, even with such actions, it is probable that the dangerous level of atmospheric GHGs will be passed, at least temporarily. We have presented evidence (Hansen et al. 2006b) that the dangerous level of CO₂ can be no more than approximately 450ppm. Our present discussion, including the conclusion that slow feedbacks (ice, vegetation and GHG) can come into play on century time-scales or sooner, makes it probable that the dangerous level is even lower.

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.

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 fibres⁵ grown without excessive fertilization that produces N₂O or other offsetting GHG emissions, it will provide continuing drawdown of atmospheric CO₂.