CHAPTER 15

THE EARTH’S CHANGING CLIMATE

CHAPTER 15

THE EARTH’S 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 4.5°C greater in 2100 than it 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 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

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

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 earth’s 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 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…

US Climate Change Science Program (2006)US Climate Change Science Program (2006)

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

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

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

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

Fig. 15.17, p. 449

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

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 an

interaction between science and policy, but science can’t 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. 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.

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

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): 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

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

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