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  • CHAPTER 15


  • In the simplest terms, climate is the average of the weatherClimate is what we expect, weather is what we get Mark TwainClimate also includes the statistics of the weather: not only the average, but the variability, and the extremesExample of a weather forecast: it will be 91 and sunny on WednesdayExample of a climate forecast: There is a 40% probability that the average temperature in College Station will be below normal in May, June, and JulyOr, a longer time in the future: the global average temperature will be 1.5 to 4.5C greater in 2100 than it was in 1990

  • What might cause these current conditions to change?

  • A forcing is a change to the global balance of radiation: it leads to more or less coming in, or more or less going outNaturalVolcanic ActivityChanges in solar outputChanges in earths orbitNatural changes in greenhouse gas concentrationsOcean currentsAnthropogenic (human-caused)Greenhouse gasesAerosolsLand-use change

  • On the left is a photograph of Muir Glacier taken on August 13, 1941, by glaciologist William O. Field; on the right, a photograph taken from the same vantage on August 31, 2004, by geologist Bruce F. Molnia of the United States Geological Survey (USGS).

  • No direct measurements of temperature, etc. except in the last ~150 yearsScientists have reconstructed the characteristics of past climates using fossils, ocean sediments, ice cores, tree rings, glacial sediments, etc.These reconstructions show a cycle of ice ages and interglacials, which are most prevalent in the northern hemisphere

  • Milankovitch theory: variations in earths orbitEccentricity (period of 100,000 years)

    Axial tilt (period of 41,000 years)More tilt = more variation between winter and summerWhen tilt is smaller, NH winters are warmer (more snow) and summers are cooler (less melting) glaciersPrecession (period of 23,000 years)Wobble of earths axisCurrently we are closer to sun in January NH winters are slightly milder and summers cooler than if we were closer in July

  • When cycles match up in certain ways, ice ages can developNorthern hemisphere ice age most likely when:Eccentricity is large (more elliptical)Tilt is smallPrecession is such that we are closer to sun in JanuaryOther factors must also play role in ice ages:Changes in CO2Changes in ocean circulationIce albedo effect

  • After the last glacial period, temperatures warmed to what is known as the Holocene Maximum (aka the climatic optimum: plants grew very well during this period)More recently, there was a little ice age in the 1500s-1800s; 1816 is known as the year without a summerSince 1900, temperatures have undergone a sharp increase

    Thousands of yearscenturiesdecades

  • Volcanic activityVolcanoes emit ash into the stratosphere, which keeps out solar radiation and cools the planetVariations in solar outputSolar energy changes with the sunspot cycleHuman activitiesIncreasing greenhouse gasesEmission of sulfates can keep out solar radiation (like with volcanoes)Other aerosols have direct and indirect effects on clouds

  • Instead of all the terrestrial (longwave) radiation escaping out to space, much of it is absorbed by gases (such as water vapor, carbon dioxide, methane) in the atmosphereThe atmosphere then radiates in all directions, and some of it comes back to the surfaceWhen this is accounted for, we can calculate the average temperature of 288 KThe atmospheric greenhouse effect is the reason the earths temperature is suitable for life

  • The primary greenhouse gas is water vapor (60%), with CO2 being the second most important (26%)CO2 concentrations have increased about 30% since the industrial revolution; surface temperatures have warmed around 1F over the past 100 yearsThis part of the equation is well understood: if greenhouse gases are increased, more longwave radiation will be absorbed and emitted back to the surface instead of escaping to space, leading to warmingHowever, the system has many complexities

  • US Climate Change Science Program (2006)



  • The forcings in the previous slides dont consider feedbacksPositive feedback: initial change is reinforced/enhancedNegative feedback: initial change is counteractedExample of positive feedback: Increased greenhouse gases warming at surface evaporation of more water vapor enhancement of greenhouse effect even more warmingExample of negative feedback: Increased greenhouse gases increased plant growth plants take CO2 out of the air decrease in greenhouse gas concentrationsSome of these feedbacks are not very well understood and cause difficulty in predicting future changes

  • Eight of the 10 warmest years since 1860 have occurred in the last decade; 1998 and 2005 are thought to be the warmest in the last 1000 yearsThe rate of warming slowed somewhat between 2005-2009March 2010 was the warmest March on record globally

  • Theory says that surface temperatures should rise when greenhouse gas concentrations increase There are many lines of independent evidence showing warming in the 20th and 21st century, as well as observations of changes in radiation due to greenhouse gasesClimate models run without increased greenhouse gases do not replicate the warming over the past 150 yrs; when GHGs are added, the model results line up with observations

  • Fig. 15.17, p. 449

  • Most models and most scientists believe that there will be continued warming for the next centurycurrent projections are for a 1.1 to 6.4C average increase by 2100 (from 19801999 averages), depending on future emissionsThere is, of course, still plenty of uncertainty due to feedbacks, poorly understood forcings, etc.Yet these aspects we dont understand well doesnt invalidate the things we do understand well!Studies of regional impacts are only starting

  • Fig. 15.18, p. 450

  • How certain do we need to be?The global effects are generally well known, but regional impacts remain uncertainWhats our tolerance for a changed climate, relative to other concerns?What, if anything, should we do about this?If we decide to limit CO2, how?Carbon tax? Cap and trade?These, and many other questions are an interaction between science and policy, but science cant provide all the answers

  • Greenhouse gases are increasing due to fossil-fuel and biomass burning280 to 380 ppm since pre-industrial, up 35%. Highest in 650,000 years at least.Aerosols increasing due to industrial activity.Earths Temp up 1.2F in past century, mostly in 1920 to 1950 and then starting in 1975. Sea Level up 2.7 inches in past 40 years, an inch in the last 10.Arctic Sea Ice decreased by 15-20% since 1978.

  • Global temp now highest in at least 500-1000 yearsGlobal temp variability due to four factors:Variability of solar outputVolcanic eruptionsAnthropogenic Sulfate AerosolsGreenhouse gasesLast 30 year dramatic warming due to greenhouse gasesWithout controlling greenhouse gas emissions, global temp will rise 2.5 to 9F over the next century

  • 6 to 16 inches of sea level rise in next century, unless Greenland goes ouch - 20 feet!Rainfall in concentrated eventsDrought and Flood increaseHurricanesMore Powerful (already see)Maybe less frequent

  • Intergovernmental Panel on Climate Change (IPCC): http://www.ipcc.chUS Global Change Research Program: are lots of blogs, some credible, some notA nice one Ive discovered recently is: It shows the scientific (rather than political or emotional) arguments for climate change. In that sense, it comes from the pro-global-warming side, but is very balanced in its presentation

  • GEOS 210 (2/3 science, 1/3 economics and policy)GEOS 410 (2/3 Public policy and economics, 1/3 science)

    Briefly mention Milantkovich*Projected surface air temperature changes from dierent climate models. Model input from greenhouse gases only is shown in yellow; input from greenhouse gases plus sulfate aerosols is shown in blue; input from greenhouse gases, sulfate aerosols, and solar energy changes is shown in red. The gray line shows observed surface temperature change. The dashed line is the 1880 to 1999 mean temperature. (Redrawn from The Science of Climate Change by Tom M. L. Wigley, published by the Pew Center of Global Climate Change.) Global average projected surface air temperature changes (C) above the 19801999 average (dark purple zero line) for the years 2000 to 2100. Temperature changes inside the graph and to the right of the graph are based on multi-climate models with different scenarios. Each scenario describes how the average temperature will change based on different concentrations of greenhouse gases and various forcing agents. The black line shows global temperature change during the 20th century. The orange line shows projected temperature change where greenhouse gas concentrations are held constant at the year 2000 level. The vertical gray bars on the right side of the figure indicate the likely range of temperature change for each scenario. The thick solid bar within each gray bar gives the best estimate for temperature change for each scenario. (Source: Climate Change 2007, The Physical Science Basis, by the Working Group 1 contribution to the Fourth Assessment Report to the IPCC 2007. Reprinted by permission of the Intergovernmental Panel on Climate Change.)


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