Last December’s meeting of the American Geophysical Union featured three of the world’s leading climate scientists: James Hansen (NASA’s chief climate scientist), Elco Rohling (National Oceanography Centre, Southampton) and Ken Caldeira (Stanford School of Earth Science). But it was Hansen who attracted the most attention when he stated:
“If you doubled CO₂, which practically all governments assume we’re going to do, that would eventually get us to the ice-free state” and “We would be sending our climate back to a state we haven’t adjusted to as a species\”.
Reaching ice-free-Earth conditions due to the addition of a few hundred parts per million CO₂ may sound like a science fiction story. But Hansen’s statement is consistent with the natural laws of physics (the Planck, Stefan-Boltzmann and Krichhoff laws of black body radiation), with atmospheric science and with the geological record.
A planet’s surface temperature is determined by the infrared absorption/emission characteristics of its atmosphere, determined by greenhouse molecules (CO₂, CH₄, N₂O, O₃). Earth’s surface conditions (including the atmospheric pressure, temperature and gases in its atmosphere) occupy an intermediate position between those of Mars and Venus. Advanced life on Earth is controlled by the presence of water and by the carbon and oxygen cycles.
Studies of the evolution of the terrestrial atmosphere based on multiple proxies (carbon isotopes in phytoplankton and in fossil soils, plant leaf stomata pores, boron isotopes, boron/calcium ratios) confirm the upper stability boundary of the Antarctic ice sheet at about 500+/-50 ppm CO₂. Other estimates suggest 615 ppm CO₂ or near-800 ppm CO₂.
The original decline in temperature from the end-Eocene (~34 million years ago) and the onset of the Antarctic ice sheet occurred when CO₂ levels declined to below ~600 ppm (as shown in Figure 1). Greenhouse gases have increased by near 40% since 1750 (from ~280 to 392 ppm CO₂, at a rate increasing to ~2.6 ppm/year by 2010). At the current rate of increase, the climate could return to greenhouse Earth conditions within 50 to 200 years.
With current emissions growing by 5.9% in 2010 (see Figure 2) and a corresponding rise of temperature by 6.2% during the last decade (see Figure 3), Earth may be committed to an ice-free state.
Climate change projections are complicated by the extreme rates of these processes. There is no precedent for such rates in the geological record, bar major greenhouse gas release triggered by methane eruptions, volcanic eruptions and asteroid impacts.
Further warming of the Greenland ice sheet and of the west and east Antarctic ice sheets may lead to pulses of ice-melt water which will cool adjacent ocean basins. Such pulsations occurred repeatedly in the North Atlantic Ocean around 8.2 thousand years ago (the Holocene Optimum), 12.9-11.7 thousand years ago (the “Youngest dryas” cold phase), and cold phases associated with the peak of earlier interglacials.
The bulk of the continents continue to heat, due to a rise in greenhouse gases, feedbacks from fires, methane release from permafrost and reduction of CO₂ intake by warming oceans.
The current consequences of polar temperature rises by 4°C and higher (see Figure 4) for the Greenland and the Antarctic ice sheets are shown in Figure 5. Between 2002 and 2008 a total of near-2500 billion tons of ice was lost while the projected rate of mass loss near-doubled over the period.
The resultant ocean-land temperature polarity generates storms, reflected in the title of James Hansen’s book, “Storms of my grandchildren”. Similar conditions developed in November 2010 as north Siberia and Canada warmed to above 4°C relative to 1951-1980 while snow storms occurred in the North Atlantic.
As atmospheric CO₂ is reaching a level unknown for the last three million years, the disconnection between science and the human response is growing. Despite warnings over the last 30 years, we are still developing global infrastructures to extract every economically accessible ton of coal, barrel of conventional or shale/sand oil and cubic meter of natural gas and coal-seam gas.
Contrarian claims by sceptics, misrepresenting direct observations in nature and ignoring the laws of physics, have been adopted by neo-conservative political parties. A corporate media maintains a “balance” between facts and fiction. The best that governments seem able to do is devise cosmetic solutions, or promise further discussions, while time is running out.
Good planets are hard to come by.