Climate Change Natural and Anthropogenic Forcing www ozeanklima

Climate Change: Natural and Anthropogenic Forcing www. ozean-klima. de WWW. OZEAN-KLIMA. DE Prof. Stefan Rahmstorf Potsdam Institute for Climate Impact Research

WWW. OZEAN-KLIMA. DE Natural Causes of Climate Change p Plate tectonics millions of years p Changes in orbit (Milankovich cycles) tens of thousands of years p Solar variability decade to centuries p “Events” – volcanic eruptions, meteorites, …

WWW. OZEAN-KLIMA. DE Climate Change on Tectonic Time Scales Royer et al. 2004

WWW. OZEAN-KLIMA. DE Orbital Climate Change: Glacial Cycles 2004 Anthropogenic rise pushes Earth system outside its Quaternary operating range 1959 290 190 4

WWW. OZEAN-KLIMA. DE Orbital Climate Change: Glacial Cycles

WWW. OZEAN-KLIMA. DE Orbital Climate Change: Glacial Cycles Model Calov&Ganopolski, 2003

WWW. OZEAN-KLIMA. DE Abrupt Warm Event 55 Myr ago Evidence of massive carbon release 55 million years ago leading to rapid climate change (Zachos et al. , 2001) 7

WWW. OZEAN-KLIMA. DE Abrupt Events During Last Glacial Dansgaard-Oeschger Events -GISP 2 ice core, Greenland 1716 14 15 12 13 1110 8 9 76 5 4 3 2 Holocene Millennia before present Up to 10 ºC warming within a decade p Likely mechanism: ocean circulation shifts p 8 d 18 O (per mil) 1

WWW. OZEAN-KLIMA. DE Temperature deviation (°C) The Last Millennium Data (Mann et al. ) Year 9

WWW. OZEAN-KLIMA. DE Temperature deviation (°C) The Last Millennium Data (Mann et al. ) Model (Bauer et al. ) Spörer minimum Maunder minimum Dalton minimum Year Good agreement of simulation and data (~ 0. 1 °C) p Solar effect in 20 th C: 0. 13 °C; greenhouse gases 0. 59 °C p 10

WWW. OZEAN-KLIMA. DE Temperature deviation (°C) The Last Millennium Data (Mann et al. ) Model (Bauer et al. ) Upper and lower IPCC range Spörer minimum Maunder minimum Dalton minimum Year Good agreement of simulation and data (~ 0. 1 °C) p Solar effect in 20 th C: 0. 13 °C; greenhouse gases 0. 59 °C p 11

WWW. OZEAN-KLIMA. DE Recent Global Warming 1990 s warmest decade p 1998, 2002, 2003, 2001 warmest years on record p 12 Surface temperature increase of ~0. 6 o. C over 20 th Century

WWW. OZEAN-KLIMA. DE Anthropogenic Forcing CO 2 concentration/ppm 380 Mauna Loa Observatory 360 340 320 300 280 Ice cores 260 1700 1800 1900 2000 Methane concentration/ppb Source: IPCC Year Carbon dioxide: 33% rise 13 1800 Flasks 1600 1400 1200 1000 Ice cores 800 600 1700 1800 1900 Year Methane: 100% rise 2000

WWW. OZEAN-KLIMA. DE Possible Natural Forcings Sunspot Number 1850 p 2000 Cosmic Rays 14 1950 2000 No trend since 1940 in possible natural forcings: solar activity, cosmic rays, volcanic activity, …

WWW. OZEAN-KLIMA. DE Recent Natural & Anthropogenic Forcing 1. 0 Observations (in black) Natural (model) Temperature change (°C) 0. 5 0. 0 – 0. 5 1. 0 Anthropogenic (model) Anthropogenic factors (GHGs and aerosols) explain recent warming 0. 5 0. 0 – 0. 5 1. 0 Natural + Anthropogenic (model) Sum of natural and human factors gives a good fit to the observations 0. 5 0. 0 15 Natural factors (solar variability and volcanoes) explain mid-century warming – 0. 5 1860 1880 1900 1920 1940 Date (year) 1960 1980 2000

WWW. OZEAN-KLIMA. DE How do we know it’s anthropogenic? Well-understood physics of greenhouse effect (since Arrhenius 1896) p No recent trends in natural forcing p Warming very unusual in terms of climate history p Attribution studies based on patterns of climate change p 16

WWW. OZEAN-KLIMA. DE 20 Global temperature change (°C) Anthropogenic Emissions (Gt. C/yr) emissions (Gt. C/yr) Anthropogenic 2 CO The Future: Stabilisation vs. BAU 4 3 2 1 0 15 1900 10 17 Unmitigated emissions 5 0 2000 2050 2100 2150 2200 2250 2300 2350 2000 2100 750 ppm stabilisation 2200 550 ppm stabilisation

WWW. OZEAN-KLIMA. DE A Few Key Points Climate has changed due to natural causes on many time scales p Past climate changes confirm what we know about climate sensitivity p Warming in past 50 years cannot be explained by natural causes, but by anthropogenic forcing p Warming in next 50 years can be halved by mitigation efforts p 18