Lecture 36 Anthropogenic Effects on Climate It is well documented that globally averaged land and sea surface temperatures have increased 0 5 C in the last century Is this the beginning of manmade global warming Two major anthropogenic forcings on global climate have been identified 1 Greenhouse gas concentrations are increasing 2 Aerosol concentrations are increasing The greenhouse effect EOM 2 10 If the earth s atmosphere were dry we could predict fairly confidently that doubling pre industrial CO2 likely by 2100 would increase mean surface temperature about 2 C The famous 19th century Swedish chemist Arrhenius was the first to predict greenhouse warming Greenhouse gas concentrations are increasing Hartmann fig 8 6 Currently 7 gigatons yr of CO2 produced by burning of fossil fuel 80 and deforestation 20 Half accumulates in atmosphere where it has a residence time of 50 years If we totally stopped burning fossil fuels forests it would take 50 years for CO2 to return half way to preindustrial levels Other anthropogenic greenhouse gasses CFC s Courtesy of Dennis Hartmann based on IPCC 1995 Methane also increasing IPCC 95 Aerosols Minute hygroscopic water attracting particles usually 0 01 1 micron in diameter Source is at surface so most aerosol is in troposphere and is washed out in days weeks by precipitation Natural dust salt soot fires sulfate ocean phytoplankton volcanoes Manmade sulfate coal oil combustion soot organics industrial processes wood burning spraying Aerosols and Climate Aerosols reflect more sunlight back to space cooling surface temperatures Some aerosols soot smoke absorb sunlight warming the atmosphere Kuwait oil fires Sulfur rich volcanic eruptions inject sulfate aerosol into the stratosphere Pinatubo 1991 20 million tons Tambora 1815 hundred million tons where it survives for years global cooling of 0 5 C Pinatubo or more Tambora in next year Sulfates soot comprise Extra Sunlight reflected back to space 4 1 the industrial haze that often blankets the eastern US Europe in summer Winds transport this haze over central Asia oceans Hartmann Indirect Effect of Aerosols Aerosols are the nucleii on which cloud droplets form More aerosol particles fi water in clouds is divided into more smaller droplets Combined surface area of droplets is larger increasing cloud reflectivity and cooling climate Striking example ship tracks Hartmann Estimates suggest 100 anthropogenic increase in nucleii over source areas 25 over remote NH oceans from wind transport of aerosols and imply indirect effect is as big as direct effect Overall effect of aerosol is to cancel 20 50 of the radiative impact of greenhouse gas Large uncertainty about aerosol absorption indirect effect If man stopped producing aerosol would take only a week or two to approach pre industrial levels Projections of Greenhouse Emissions Without mitigation greenhouse emissions rise IPCC 2001 Even if emissions are stabilized at current levels CO2 reaches double preindustrial values by 2100 Sulfate aerosol which may be reining back global warming to date is already starting to drop due to air pollution concerns Rapidly increasing human greenhouse effect one scenario shown Climate Feedbacks The greenhouse effect is indisputable Nevertheless even if we perfectly knew future CO2 there is some uncertainty about how much warming and how fast it will lead to due to natural climate feedbacks Water vapor positive Clouds unclear Snow and ice cover positive Ocean circulation delays Water vapor likely a positive feedback Water vapor is a balance between surface evaporation faster at low RH and precipitation requires high RH maintaining an equilibrium pattern of RH At higher temperatures more water vapor is required to produce this RH pattern CO2 temp water vapor greenhouse Other feedbacks temp snow and ice albedo Clouds are complex to model feedback uncertain Oceans slower to warm short term heat sink Global climate models GCMs interactively coupled to computer models of ocean circulation simulate these climate feedbacks Weather forecast models with coarser grid spacing of 200 500 km can simulate 100 or more years Careful representations of relevant physical processes clouds radiation surface characteristics sea ice etc Tested on past current climate seasonal march weather In US two main models are run at NCAR Boulder CO and GFDL Princeton GCM predictions have uncertainties and different plausible models produce slightly different results but are our best current guess at climate change in next 100 years Are climate models consistent with observed changes to date Krakatoa Predictions of Climate Change in Next 100 Years While many uncertainties best current information suggests climate will warm alarmingly 2 5 C global average larger toward poles in next 100 years unless strong measures are taken to curb fossil fuel consumption greenhouse gas emission US 5 of world population creates 25 of problem now IPCC 2001 Predicted warming is much larger in polar regions Other global climate concerns CO2 induced ocean acidification dissolves calcium shelled marine organisms Bleaching coral reefs due to warming of oceans Large poleward migration of natural ecosystems e g West Nile virus Warming arctic bogs and frozen clathrates may become source of methane CO2 remains in the system for 1000 years Sea level rise and glacial melt continue for many centuries with sizeable chance of melting Greenland icecap over the next 1000 years which would add 20 ft of sea level rise Climate surprises yet to be appreciated Climate Change in the Northwest Scenario 1 per year CO2 increase 1990 UW CIG Results from one model UW CIG Each 2 C 3 5 F of warming causes snowline to rise by 300 m 1000 ft 2500 2000 5500 2100 Predicted snowpack reduction 50 by 2050 March Impacts Summer water Forests fires Salmon Skiing June Base 2025 2095 mm water equiv 0 100 200 300 400 500 600 700 800 900 So what are we to do Precaution suggests that we try to stabilize climate before enormous changes occur a giant ship is coming adrift Internationally greenhouse warming is recognized as a serious problem Kyoto protocol would have been a start but US govt has not attempted to implement an energy policy aimed at major cuts in greenhouse emissions We should support leaders and organizations at all levels of government and society who recognize the seriousness of climate change and the need for starting an organized transition away from fossil fuels now