1. CHAPTER 15 THE EARTH’S CHANGING CLIMATE CHAPTER 15 THE EARTH’S CHANGING CLIMATE

2.  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 4.5°C greater in 2100 than it was in 1990

3. What might cause these current conditions to change?

4.  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 earth’s orbit ◦ Natural changes in greenhouse gas concentrations ◦ Ocean currents  Anthropogenic (human-caused) ◦ Greenhouse gases ◦ Aerosols ◦ Land-use change

5.  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).

6.  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

7.  Milankovitch theory: variations in earth’s 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 earth’s axis  Currently we are closer to sun in January – NH winters are slightly milder and summers cooler than if we were closer in July

8.  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

9.  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

10.  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

11.  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 earth’s temperature is suitable for life

12.  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 1°F 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…

14. US Climate Change Science Program (2006)

15. http://www.ipcc.ch/graphics/ar4-wg1/jpg/fig-2-20.jpg

17.  The forcings in the previous slides don’t 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

18.  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 2005- 2009  2010 tied 2005 as the warmest record globally!  While you were in this class, we had the warmest September on record globally. http://www.ncdc.noaa.gov/img/climate/research/ global-jan-dec-error-bar-pg.gif http://www.ncdc.noaa.gov/sotc/get- file.php?report=global&file=map-land-sfc- mntp&year=2010&month=3&ext=gif

19.  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 observations http://www.ipcc.ch/publications_and_data/ar4/ wg1/en/faq-9-2-figure-1.html

20. Fig. 15.17, p. 449

24.  Most models and most scientists believe that there will be continued warming for the next century–current projections are for a 1.1 to 6.4ºC average increase by 2100 (from 1980—1999 averages), depending on future emissions  There is, of course, still plenty of uncertainty due to feedbacks, poorly understood forcings, etc.  Yet these aspects we don’t understand well doesn’t invalidate the things we do understand well!  Studies of regional impacts are only starting http://www.ipcc.ch/publications_and_data/ar4/wg1/en/fi g/figure-spm-5.jpeg

25. Fig. 15.18, p. 450

27.  How certain do we need to be? ◦ The global effects are generally well known, but regional impacts remain uncertain ◦ What’s 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 can’t provide all the answers…

28.  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.  Earth’s Temp up 1.2°F 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.

29.  Global temp now highest in at least 500-1000 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 9°F over the next century

30.  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

31.  Intergovernmental Panel on Climate Change (IPCC): http://www.ipcc.chhttp://www.ipcc.ch  US Global Change Research Program: http://globalchange.gov/ http://globalchange.gov/  There are lots of blogs, some credible, some not…  A nice one I’ve discovered recently is: www.skepticalscience.com 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 www.skepticalscience.com

32.  GEOS 210 (2/3 science, 1/3 economics and policy)  GEOS 410 (2/3 Public policy and economics, 1/3 science)