CHAPTER 15
THE EARTH’S CHANGING CLIMATE
CHAPTER 15
THE EARTH’S CHANGING CLIMATE
In the simplest terms, climate is the average of theweather
◦“Climate is what we expect, weather is what we get” – MarkTwain
Climate also includes the statistics of the weather:not only the average, but the variability, and theextremes
Example of a weather forecast: it will be 91 andsunny on Wednesday
Example of a climate forecast: There is a 40%probability that the average temperature in CollegeStation will be below normal in May, June, and July
Or, a longer time in the future: the global averagetemperature will be 1.5 to 4.5°C greater in 2100 thanit was in 1990
What might cause these current conditions to change?What might cause these current conditions to change?
A forcing is a change to the global balance ofradiation: it leads to more or less coming in, ormore or less going out
Natural
◦Volcanic Activity
◦Changes in solar output
◦Changes in earth’s orbit
◦Natural changes in greenhouse gasconcentrations
◦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 photographtaken from the same vantage on August 31, 2004, by geologistBruce 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 ofpast climates using fossils, ocean sediments, icecores, tree rings, glacial sediments, etc.
These reconstructions show a cycle of ice ages andinterglacials, which are most prevalent in thenorthern hemisphere
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 areslightly milder and summers cooler than if we were closerin July
When cycles match up incertain ways, ice ages candevelop
Northern hemisphere ice agemost likely when:
◦Eccentricity is large (moreelliptical)
◦Tilt is small
◦Precession is such that weare closer to sun in January
Other factors must also playrole in ice ages:
◦Changes in CO2
◦Changes in ocean circulation
◦Ice albedo effect
After the last glacialperiod, temperatureswarmed to what isknown as the HoloceneMaximum (aka theclimatic optimum:plants grew very wellduring this period)
More recently, there wasa “little ice age” in the1500s-1800s; 1816 isknown as the “yearwithout a summer”
Since 1900,temperatures haveundergone a sharpincrease
Thousands
of years
centuries
decades
Volcanic activity
◦Volcanoes emit ash into the stratosphere, whichkeeps 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 onclouds
Instead of all the terrestrial(longwave) radiationescaping out to space, muchof it is absorbed by gases(such as water vapor, carbondioxide, methane) in theatmosphere
The atmosphere thenradiates in all directions, andsome of it comes back to thesurface
When this is accounted for,we can calculate the averagetemperature of 288 K
The atmospheric greenhouseeffect is the reason theearth’s temperature issuitable for life
The primary greenhouse gas is water vapor(60%), with CO2 being the second mostimportant (26%)
CO2 concentrations have increased about 30%since the industrial revolution; surfacetemperatures have warmed around 1°F over thepast 100 years
This part of the equation is well understood: ifgreenhouse gases are increased, morelongwave radiation will be absorbed andemitted back to the surface instead of escapingto space, leading to warming
However, the system has many complexities…
US Climate Change Science Program (2006)US Climate Change Science Program (2006)
http://www.ipcc.ch/graphics/ar4-wg1/jpg/fig-2-20.jpg
http://www.ipcc.ch/graphics/ar4-wg1/jpg/fig-2-20.jpg
The forcings in the previous slides don’t considerfeedbacks
◦Positive feedback: initial change isreinforced/enhanced
◦Negative feedback: initial change is counteracted
Example of positive feedback: Increasedgreenhouse gases warming at surface evaporation of more water vapor enhancementof greenhouse effect even more warming
Example of negative feedback: Increasedgreenhouse gases increased plant growth plants take CO2 out of the air decrease ingreenhouse gas concentrations
Some of these feedbacks are not very wellunderstood and cause difficulty in predicting futurechanges
Eight of the 10 warmestyears since 1860 haveoccurred in the last decade;1998 and 2005 are thoughtto be the warmest in the last1000 years
The rate of warming slowedsomewhat between 2005-2009
2010 tied 2005 as thewarmest record globally!
While you were in this class,we had the warmestSeptember on recordglobally.
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
Theory says that surfacetemperatures should risewhen greenhouse gasconcentrations increase
There are many lines ofindependent evidenceshowing warming in the20th and 21st century, aswell as observations ofchanges in radiation dueto greenhouse gases
Climate models runwithout increasedgreenhouse gases do notreplicate the warmingover the past 150 yrs;when GHGs are added,the model results line upwith observations
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-9-2-figure-1.html
Fig. 15.17, p. 449
Most models and mostscientists believe that therewill be continued warmingfor the next century–currentprojections are for a 1.1 to6.4ºC average increase by2100 (from 1980—1999averages), depending onfuture emissions
There is, of course, stillplenty of uncertainty due tofeedbacks, poorlyunderstood forcings, etc.
Yet these aspects we don’tunderstand well doesn’tinvalidate the things we dounderstand well!
Studies of regional impactsare only starting
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/fig/figure-spm-5.jpeg
Fig. 15.18, p. 450
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 aninteraction between science and policy, butscience can’t provide all the answers…
Greenhouse gases are increasing due to fossil-fueland biomass burning
◦280 to 380 ppm since pre-industrial, up 35%. Highest in650,000 years at least.
Aerosols increasing due to industrial activity.
Earth’s Temp up 1.2°F in past century, mostly in1920 to 1950 and then starting in 1975.
Sea Level up 2.7 inches in past 40 years, an inch inthe last 10.
Arctic Sea Ice decreased by 15-20% since 1978.
Global temp now highest in at least 500-1000years
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 greenhousegases
Without controlling greenhouse gas emissions,global temp will rise 2.5 to 9°F over the nextcentury
6 to 16 inches of sea level rise in nextcentury, unless …
◦Greenland goes – ouch - 20 feet!
Rainfall in concentrated events
Drought and Flood increase
Hurricanes
◦More Powerful (already see)
◦Maybe less frequent
There are lots of blogs, some credible, somenot…
A nice one I’ve discovered recently is:www.skepticalscience.com It shows the scientific(rather than political or emotional) arguments forclimate change. In that sense, it comes from the“pro-global-warming” side, but is very balancedin its presentation
GEOS 210 (2/3 science, 1/3 economics andpolicy)
GEOS 410 (2/3 Public policy and economics,1/3 science)
