CHAPTER 15 THE EARTH’S CHANGING CLIMATE

1. CHAPTER 15 THE EARTH’S CHANGING CLIMATE

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

3. Current Climate Regions What might cause these current conditions to change?

4. Climate Forcings • 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. Now impossible to refute warming… • 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. Past climate • 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. What causes changes on shorter timescales? • 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

8. Refresher: Greenhouse effect • 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

9. Enhancement of greenhouse effect • 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 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…

11. Summary of forcings US Climate Change Science Program (2006)

12. Summary of forcings http://www.ipcc.ch/graphics/ar4-wg1/jpg/fig-2-20.jpg

13. Feedbacks • 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 CO2 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

14. Recent warming • 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 • March 2010 was the warmest March 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

15. Evidence for anthropogenic warming • 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 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

16. Fig. 15.17, p. 449

17. Projections for future warming • 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/fig/figure-spm-5.jpeg

18. Fig. 15.18, p. 450

19. Many levels of questions… • 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…

20. What We Know For Sure • 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.

21. What we are fairly certain about • 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

22. More debate involved: impacts • 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

23. For further reading… • Intergovernmental Panel on Climate Change (IPCC): http://www.ipcc.ch • US Global Change Research Program: 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

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