CHAPTER 15 THE EARTHS CHANGING CLIMATE CHAPTER 15 THE EARTHS CHANGING CLIMATE In the simplest terms, climate is the average of the weather Climate is what we expect, weather is what we get Mark Twain Climate also includes the statistics of the weather: not only the average, but the variability, and the extremes Example of a weather forecast: it will be 91 and sunny on Wednesday Example of a climate forecast: There is a 40% probability that the average temperature in College Station will be below normal in May, June, and July Or, 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 out Natural Volcanic Activity Changes in solar output Changes in earths orbit Natural changes in greenhouse gas concentrations Ocean currents Anthropogenic (human-caused) Greenhouse gases Aerosols Land-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 years Scientists 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 orbit Eccentricity (period of 100,000 years) Axial tilt (period of 41,000 years) More tilt = more variation between winter and summer When tilt is smaller, NH winters are warmer (more snow) and summers are cooler (less melting) glaciers Precession (period of 23,000 years) Wobble of earths axis Currently 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 develop Northern hemisphere ice age most likely when: Eccentricity is large (more elliptical) Tilt is small Precession is such that we are closer to sun in January Other factors must also play role in ice ages: Changes in CO 2 Changes in ocean circulation Ice 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 summer Since 1900, temperatures have undergone a sharp increase Thousands of years centuries decades Volcanic activity Volcanoes emit ash into the stratosphere, which keeps out solar radiation and cools the planet Variations in solar output Solar energy changes with the sunspot cycle Human activities Increasing greenhouse gases Emission 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 atmosphere The atmosphere then radiates in all directions, and some of it comes back to the surface When this is accounted for, we can calculate the average temperature of 288 K The atmospheric greenhouse effect is the reason the earths temperature is suitable for life The primary greenhouse gas is water vapor (60%), with CO 2 being the second most important (26%) CO 2 concentrations have increased about 30% since the industrial revolution; surface temperatures have warmed around 1F over the past 100 years This 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 warming However, the system has many complexities US Climate Change Science Program (2006) The forcings in the previous slides dont consider feedbacks Positive feedback: initial change is reinforced/enhanced Negative feedback: initial change is counteracted Example of positive feedback: Increased greenhouse gases warming at surface evaporation of more water vapor enhancement of greenhouse effect even more warming Example of negative feedback: Increased greenhouse gases increased plant growth plants take CO 2 out of the air decrease in greenhouse gas concentrations Some 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 years The rate of warming slowed somewhat between 2010 tied 2005 as the warmest record globally! While you were in this class, we had the warmest September on record globally.global-jan-dec-error-bar-pg.giffile.php?report=global&file=map-land-sfc- mntp&year=2010&month=3&ext=gif Theory says that surface temperatures should rise when greenhouse gas concentrations increase There are many lines of independent evidence showing warming in the 20 th and 21 st century, as well as observations of changes in radiation due to greenhouse gases Climate 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 observationswg1/en/faq-9-2-figure-1.html Fig , 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 emissions There 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 startingg/figure-spm-5.jpeg Fig , p. 450 How certain do we need to be? The global effects are generally well known, but regional impacts remain uncertain Whats 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 burning 280 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 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 years Global temp variability due to four factors: Variability of solar output Volcanic eruptions Anthropogenic Sulfate Aerosols Greenhouse gases Last 30 year dramatic warming due to greenhouse gases Without 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 events Drought and Flood increase Hurricanes More Powerful (already see) Maybe less frequent Intergovernmental Panel on Climate Change (IPCC): US Global Change Research Program: There are lots of blogs, some credible, some not A 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)