Dangerous Climate Change: Tipping Points and Radical Geoengineering. Peter Cox Professor of Climate System Dynamics University of Exeter

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1 Dangerous Climate Change: Tipping Points and Radical Geoengineering Peter Cox Professor of Climate System Dynamics University of Exeter United Nations Framework Convention on Climate Change (UNFCCC) The ultimate objective [is]. stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system Introduces the notion of Dangerous Climate Change.but how can this be defined? 1

2 Definitions of Tipping Point The tipping point is the.critical point..at which the future state of the system can be switched into a qualitatively different state by small perturbations (based on Lenton et al., 2008) when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause (Abrupt Climate Change, NAS, 2002) Typical Characteristics of Systems with Tipping Points Have more than one equilibrium state. Current equilibrium becomes unstable at the Tipping Point (gain >1) Magnitude and rate of change at the Tipping Point is a system feature and is independent of the forcing. Crossing a Tipping Point may result in a new stable state, implying a degree of irreversibility or hysteresis. Many possible climate Tipping Points have now been identified. 2

3 David Stephenson University of Exeter A Mathematician Explains Tipping Points Initial equilibrium state Small Perturbation Return to Initial equilibrium state A Mathematician Explains Tipping Points Initial equilibrium state Large Perturbation Flip to New equilibrium state 3

4 Tipping Points (Lenton et al., 2008) Map of potential policy-relevant tipping elements in the climate system, updated from ref. 5 and overlain on global population density Gulf-stream Collapse Collapse of the Indian Monsoon Greening of the Sahara Lenton T. M. et.al. PNAS 2008;105: by National Academy of Sciences Collapse of the Indian Monsoon? Evidence of Monsoon Switching from Cave Stalagmites in China HIGH RAINFALL Stalagmite Record Variation in sunlight LOW RAINFALL End of Ice-Age Before Last Could pollution switch the monsoon off? 4

5 Evidence of Fertile Sahara 6-9 kyr ago Greening of the Sahara Rock Painting from Tassili N Ajjer Dynamic Simulation of Sahara through Holocene Summer insolation Rainfall Vegetation fraction Eolian dust B,C CLIMBER-2 results (Claussen et al., 1999) D proxy data (demenocal et al., 2000) 5

6 Multiple States arise from Atmosphere- Vegetation Interaction in mid-holocene Arid Sahara NO VEGETATION FEEDBACK ON ATMOSPHERE Green Sahara WITH VEGETATION FEEDBACK Claussen and Gayler, 1997 Collapse of the Thermohaline Circulation ( Gulf Stream ) Could the Gulf Stream Collapse Under Global Warming? wouldn t be quite as dramatic as the Hollywood version. but serious nonetheless 6

7 Thermohaline Circulation (Rahmstorf, Nature 2002) Surface Deep Bottom Salinity > 36 Salinity < 34 Deep Water Formation Phase Diagram for Thermohaline Circulation On State Off State Rahmstorf, Climatic Change (2000) 7

8 Interglacial-mode of the Thermohaline Circulation Heat Transport by the THC keeps the north Atlantic up to 6 o C warmer than it would otherwise be Slide Courtesy of Stefan Rahmsdorf Glacial-mode of the Thermohaline Circulation Deep-water formation further south and a cold, ice-covered North Atlantic Could human-induced climate change lead to a similar shutdown of the THC? Slide Courtesy of Stefan Rahmsdorf 8

9 Temperature ( C) d 18 O (per mil) Evidence of THC Instability? : Dansgaard-Oeshger Events -GISP2 ice core, Greenland -Sediments, subtropical Atlantic Holocene Millennia before present Up to 5ºC warming within a decade Other events: Heinrich events, 8k event, Younger Dryas Slide Courtesy of Stefan Rahmsdorf most climate models predict a weakening but not a collapse of the Gulf Stream Weaker Gulf Stream Source: IPCC TAR 9

10 COULD WE PREDICT TIPPING POINTS? How could a Hippo have predicted the Aridification of the Western Sahara 5500 years ago? Green West Sahara Desert West Sahara Marten Scheffer, Kavli CentreTipping Points meeting,

11 Not from the trend... But from the noise... Restoring force towards equilibrium becomes weaker as the Tipping Point is approached Oscillations become slower but larger..so we may be able to get some forewarning of Tipping Points 11

12 Geoengineering is... Deliberate large-scale intervention in the Earth s climate system in order to moderate global warming Why are some now seriously considering geoengineering? Projections of future climate change produce global warming between 1.1 and 6.4 o C by 2100 (IPCC TAR). To have a reasonable chance of avoiding a potentially dangerous 2 o C global warming through conventional mitigation requires global CO 2 emission cuts of about 60% by 2050 (..and continous reductions thereafter ). However, global CO 2 emissions have continued to rise. Could Geoengineering provide an alternative way to avoid dangerous climate change? 12

13 Climate Energy Balance Solar Radiation Management options Carbon Dioxide Removal options Space-based Sunshade Cloud Brightening Ships 13

14 Stratospheric Aerosol Injection to mimic the climatic effects of volcanoes.. Mt. Pinatubo, 1991 Soden et al., 2002 Volcanoes cause global cooling by putting aerosols in the stratosphere 14

15 PROS Volcano-like Geoengineering Cheap: direct costs would be very low compared to Mitigation of CO 2 emissions. Fast: to act so could be used in an emergency in response to a tipping point. Can effectively offset global warming. CONS Does not prevent regional changes in rainfall. Could delay recovery of the ozone hole. Does not deal with ocean acidification problems. Climate Model Geoengineering Experiments 15

16 Could we avoid droughts in Sub-Saharan Africa by injecting aerosols into the stratosphere? North & South>> Not Much Change in Risk North only>> Increase in Risk South only >> Big Reduction in Risk! Geoengineering Begins Conclusions Despite widespread concerns over climate change global CO 2 emissions have continued to grow, and it now looks unlikely that we can avoid at least 2 o C of global warming by cutting CO 2 emissions. There is good evidence of climate Tipping Points in the past abrupt changes that would be difficult to adapt to (e.g. collapse of the Indian monsoon or the ocean heat conveyer belt), and concern that we could cross Tipping Points in the future. We may be able to get some forewarning of tipping points through changes in the variations of the climate system, but conventional mitigation would probably too slow to avoid the tipping point even then. Some Geoengineering approaches (e.g. stratospheric aerosol injection) would be fast acting and so could be used in an emergency, but they may have undesirable side-effects. Exeter University is actively researching tipping points and geoengineering. 16