Vanishing Polar Ice Sheets

Abstract

Global temperatures have increased by 0.8°C since instrumental records began, and the last decade has been the warmest. This warming has been closely linked to increasing greenhouse gas concentrations due to human activities. Arctic warming is leading to thinning and disappearance of sea ice on the Arctic Ocean and the increased melting of the Greenland ice sheet. In the last decade the Antarctic ice sheet has also begun to respond. Shrinking ice sheets have significant implications for sea level rise. Knowledge of their behaviour in the geological past, along with climate and ice sheet modelling, provides guidance on what we can expect in the future. Over the past 50 million years (m.y.), the world has evolved from a largely ice-free Greenhouse Earth with atmospheric CO2 concentrations 3–6 times pre-industrial levels, and sea level over 60 m higher, into an Icehouse Earth with lower CO2 concentrations. Concentrations were low enough by 34 m.y. ago to reduce temperatures and allow the formation of the first big Antarctic ice sheets, and further cooling and reduction in greenhouse gases followed until the continental Northern Hemisphere ice sheets formed about 2.5 m.y. ago. Trapped gases in ice cores allow us to estimate past atmospheric CO2 levels quite accurately for the last 800,000 years. Geochemical estimates of earlier CO2 levels are much less accurate but nevertheless consistently indicate these have not exceeded 400 ppm in the last 24 m.y. Atmospheric CO2 concentration has increased from 280 ppm in the pre-industrial times to 390 ppm in early 2010. Earth’s atmosphere last had such concentrations ~3 million years ago when geochemical measurements indicate global surface temperature was ~3°C warmer than today, and global sea-level was ~25±5 m higher. This implies the loss of the Greenland ice sheet, the West Antarctic ice sheet and a part of the much larger East Antarctic ice sheet. Both field evidence and modelling studies imply rates of ice loss are slow, of the order of a meter or two of sea level equivalent per century, but there are significant uncertainties because key boundary conditions for ice sheet collapse are not well understood. Feed-back effects on atmospheric warming and ice loss could well accelerate the process. The Arctic is today warming as fast as, or faster than, any other area on earth, with the imminent loss of summer sea-ice and increasing loss of ice from Greenland (adding to rising global sea level). Antarctic warming has also begun to rise, despite its suppression from the loss of the ozone hole in the last three decades. Nevertheless the warming of shallow to intermediate Southern Ocean waters and the rise of Antarctic surface temperatures has led to the collapse of ice shelves around the Antarctic Peninsula since 1990 and in the last decade the acceleration of ice loss from West Antarctica, as well as parts of East Antarctica. The thermal inertia of oceans and ice sheets means that they take decades to centuries to respond but once ice loss has begun it could continue for millennia. The ice sheets have now begun to shrink. Both modelling and palaeodata indicate a significant risk of setting in train events leading to their inevitable loss unless the release of CO2 from fossil fuels is curtailed and CO2 sinks are found to return atmospheric CO2 levels to well below 400 ppm.