Tipping Points that Could Lead to Abrupt Climate Change IGSDIINECE Climate Briefing Note: September [], 2008 DRAFT FOR REVIEW 09/16/08 I. Introduction A. Summary The paleoclimate records show that past climate changes have included both steady, linear changes as well as abrupt, non-linear changes where small increases in global warming produced large and irreversible impacts once certain thresholds, or "tipping points", were passed. A non-linear tipping point is analogous to the final step one takes walking off a cliff. Once the step is taken, there is no going back. Abrupt climate change can produce potentially catastrophic impacts, such as the shutdown or reorganization of global currents, dieback of the Amazon rainforest, and collapse of the Greenland Ice Sheet. 1 Experts have concluded that anthropogenic forcing could increase the risk of abrupt climate change and that tipping points could be passed this century, or even this decade.2 If we continue emitting greenhouse gases in a "business-as-usual" scenario, permitting atmospheric carbon dioxide (C02) concentrations to rise 2-3 ppm every year, the question is not whether abrupt climate change will occur, but rather how soon it will occur.3 In fact, some experts conclude that 385 ppm, the current concentration of CO2, is already too high, and that we must return to 350 ppm if we want to preserve a planet similar to that on which civilization developed and to which humanity is adapted.4 Despite the certainty that abrupt changes have occurred in the past and could be triggered again in the near future, current climate change policy does not account for abrupt climate change. In particular, abrupt climate change is not incorporated into the projections of the Intergovernmental Panel on Climate Change (lPCC), which is regarded as the most authoritative, if often too conservative, source of information on climate issues.5 B. Definition of Abrupt Climate Change "Abrupt climate change" is the large-scale transformation in a climate system which occurs when it is forced past a tipping point and which could lead to potentially catastrophic impacts.6 "The term 'tipping point' commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system.,,7 It is a point at which the climate, without additional forcing, undergoes rapid changes that are practically uncontrollable. 8 Positive feedbacks, physical responses that amplify the original actions, can accelerate linear changes and push a system past a tipping point. The "point of no return" is a "climate state beyond which the consequence is inevitable, even if climate forcings are reduced.,,9 Sometimes a point of no return can be avoided even if a tipping point is exceeded, for example when greenhouse gases sufficient to raise global temperature past a critical threshold are released but their effects are delayed. 10 1 University of Hawaii School of Law Library - Jon Van Dyke Archives Collection Scientists have identified specific geographical areas, weather systems, forests, and even ocean currents that might be vulnerable to tipping points in the next century, with Arctic sea-ice, which is rapidly disappearing in summer, and the Greenland Ice Sheet, which is steadily losing mass, considered the most sensitive. II These large-scale components of the Earth system that may pass a tipping point are referred to as "tipping e1ements.,,12 In addition to Arctic sea-ice and the Greenland Ice Sheet, other tipping elements include the West Antarctic Ice Sheet, Atlantic Thermohaline Circulation, which is a global current, and Amazonian and boreal forests. Meteorological phenomena such as El Nino-Southern Oscillation and the Indian Summer Monsoon are also tipping elements affected by global temperature changes. For some of these tipping elements, there is a high degree of uncertainty as to triggers and timescales for transitions and impacts. Even for the most sensitive tipping elements, expert predictions vary as to how soon a tipping point will be passed - or if one already has been passed. The Intergovernmental Panel on Climate Change (IPCC) typically issues the most conservative estimates, some of which are considered by many experts to be out-of-date before they are even published. 13 However, the findings of the IPCC in its 2007 Fourth Assessment Report (AR4) have been frequently used as starting points or points of contrast for other experts' more current and more aggressive predictions for global warming and climate change. II. Tipping Elements A. Arctic Sea Ice In both summer and winter, the presence of Arctic sea ice has declined steadily over the last 30 years, and decreases in summer sea ice in the last 3 years have been dramatic. Less ice means less snow cover, a reduction in snow albedo, or reflectivity, and more absorption of heat by exposed Arctic waters, surface pools, and darker ice, all of which lead to further warming and ice melting. 14 The ice albedo positive feedback that converts white snow and ice into slick, dark ice is sometimes referred to as "albedo flip.,,15 Aside from this positive feedback, further darkening of polar surfaces is caused when black carbon, or soot, is released into the atmosphere and falls to earth blanketing snow and ice. 16 Whereas clean snow cover bounces 70-90% of the sun rays back into space, dirty snow can absorb more than 65% of solar radiation, while dark ice and open water absorb up to 95%.17 In the AR4, which was published in December 2007, the IPCC projected that an increase of about 9°C in the Arctic could cause a disappearance of late summer sea ice before the end of the century. IS Since warming is generally over twice as strong at the highest and lowest latitudes, this would correspond with a global temperature increase of about 4°c. 19 In late 2006 and early 2007, however, many climatologists were already predicting completely ice-free Arctic summers by as early as 2030,z° Despite these predictions, the scientific community was stunned when on September 16, 2007, the Arctic sea ice coverage decreased to 4.13 million square kilometers, compared to the previous record low of 5.32 million square kilometers in 2005,z1 2 University of Hawaii School of Law Library - Jon Van Dyke Archives Collection The 2008 summer ice is on pace to be the second-lowest on record as of September 3, 2008 [update tomorrow] when it measured 4.85 million square kilometers, making it clear that the Arctic is now headed toward irreversible sea ice loss very quickly, with a transition to an ice-free Arctic summer expected in the next decade with a local temperature increase of only 0.5_2°C?2 Alarmingly, human activity has already raised the average global temperature 0.8°C above pre-industrial levels with another 0.6° C "in the pipeline," yet to manifest but that will be caused by pollution we have already released?3 Given the more than double sensitivity of polar latitudes - meaning an increase as small as 0.2-0.250 C globally could be enough to melt the sea ice completely - some predictions have summer sea ice making its first full retreat as early as late summer 2013.24 Summer of 2008 is also the first year that the Northwest Passage over the top of North America, and the Northeast Passage over the top of Russia are both clear of ice.25 The opening of these channels will likely significantly increase emissions in high latitudes by opening a new, shorter ocean route between Europe and Asia, "a prospect already drawing billions of dollars in investment in ice-class ships".26 Many experts adjudge that we are past the point of no return for loss of Arctic summer sea ice. 27 Whether or not this is the case, when the transition occurs, the amplified warming is not only likely to heat up the regional environment - which could lead to thawing of permafrost in the Arctic tundra amongst other impacts - but also likely to hasten the melting of the Greenland Ice Sheet.28 B. Greenland Ice Sheet Recent observations of the Greenland Ice Sheet (GIS) show that surface mass is declining, coastal ice is thinning and glaciers are experiencing surges - periods of rapid advancement - all at speeds that cannot be explained by IPCC ice sheet models.29 Warming along coasts is expected to decrease surface altitude, leading to increased warming and generating a positive feedback loop which will push the GIS fast a tipping point beyond which the ice sheet will disintegrate or contract significantly.3 Accelerating the melting on the GIS is the penetration of warm ocean waters that carve ice streams into the glaciers, sometimes permeating them down to the bottom?! The melting is also triggering earthquakes across the GIS, and the large chunks of glaciers that break off are carried out to sea by the streams.32 In light of these multiple positive feedbacks, experts predict that the GIS will reach its tipping point with a local temperature rise of 2.7_3.0°C.33 Modeling the dynamics of ice streams has proved difficult for scientists, however, and not one has yet been able to create an adequate simulation of the rapid disappearance of ice sheets at the end of the last ice age. 34 Reflecting this large degree of uncertainty, the IPCC puts the tipping range for the GIS at 1.9-4.6°C global temperature raise, but even the high end of this range is easily achievable by 2100?5 Despite the lack of verifiable projections, many experts state that the GIS will probably contribute more and faster to sea level rise than demonstrated by the current models, 3 University of Hawaii School of