Global Warming

From GeoClasses

Global warming is one of the greatest environmental issues facing the world. Currently the scientific consensus is that global warming is occuring and that anthropogenic forcing (humans) are causing it, but there are still large uncertainties about the impacts and costs of dealing with the issue. This lecture will look at the science and potential impacts of global warming based primarily on the IPCC 2001 climate report.

An inconvenient truth?

• Al Gore's documentary about global warming.

Carbon dioxide and global temperatures

Glacial-interglacial correlation

This graph shows the correlation between atmospheric temperature and carbon dioxide concentration over the last million year, based on ice core data.

Over the past 5 million the Earth's climate has shifted from glacial to interglacial periods. Ice cores taken from Vostok, Antarctica have shown that the concentration of carbon dioxide in the atmosphere corelates with temperature.

Recent anthropogenic impacts on the atmosphere

Global carbon dioxide production

The invention of the steam engine 200 years ago sparked the industrial revolution, which began a switch from economies based on manual labour to ones based on mechanization. Steam engines originally required coal to heat the water (to produce the steam) that drove the turbines, but even today most of our energy is produced using fossil fuels like coal, oil and natural gas. The burning of these fossil fuels liberated massive amounts of carbon dioxide into the atmosphere.

The correlation between carbon dioxide and global temperatures seen in the cycles of glacial and interglacial periods, and the fact that carbon dioxide is a greenhouse gas, sparked concern that global warming was caused by humans.

Predicting/Modeling global warming

Comparison of computer model results (grey) to observed temperatures.

How do we know that global warming is occuring, and how do we know it is being caused by greenhouse gas emissions?

Some of the evidence comes from the past correlations, but important evidence that anthropogenic greenhouse gas emissions are causing the warming that we see today comes from computer models.

Computer models use what we know about the physics of the atmosphere (lots of equations), the geography of the Earth and the chemistry of the atmosphere to simulate global atmospheric circulation and temperatures.

• Without anthropogenic forcing the models are able to reproduce much of the pattern of global temperature change over the last 150 years, but miss the rapid temperature increase of the last 30 years.
• Models that neglect the natural atmospheric changes that affect the global climate (such as volcanic erruptions), but include the anthropogenic effects, capture the rapid temperature rise of the last 30 years very well, but miss some of the significant changes over the last century.
• Models that combine both natural and anthropogenic effects, simulate global temperatures very well. (Note: this image is based on models constructed prior to 2001. Models have improved even more since then.)

Effects of Global Warming

Climate change over the last century

Temperature

Annual temperature trends (1976-2000)

Precipitation

Annual precipitation trends 1901-2000.

Glacier retreat

Retreat of the Grinnell Glacier in Glacier National Park.

Many alpine glaciers are retreating around the world.

In the US' Glacier National Park:

The larger glaciers are now approximately 1/3 their size in 1850 (range - 23-38%) and numerous smaller glaciers have disappeared. There has been a 73% reduction in the area of Glacier National Park covered by glaciers from 1850-1993. Only 27 km2 of glaciers remain from the 99 km2 which previously existed. Out of 84 watersheds, only 18 have 1% glacier cover, 8 have 2% and 4 have 3%. Average glacier area in the accumulation zone for September 1993 was 35%, indicating negative mass balances for most glaciers and continued shrinkage.

from [USGS: Glacier monitoring in Glacier National Park]

Sea-level Rise

Paleo sea-level rise

Sea levels will change in response to global warming for two reasons.

Sea-level rise since the last glacial maximum.

• Melting of land based glaciers, such as alpine glaciers and the ice sheets on Greenland and Antarctica. The melting ice flows into the ocean raising the sea-level. Since the last glacial maximum (about 20,000 years ago) sea-level has risen by over 100 m.
• Expansion of ocean water. Water is densest at 4 degrees Celcius. As it warms above 4 deg C it expands, so a warming of ocean water will cause it to expand.

Sea-level rise in the last 100 years.

Predicted Sea-level Rise

Models of sea-level rise in the next 100 years have substantial uncertainty because we do not understand well enough the response of glaciers to warmer temperatures. Some research suggests that melting of the ice can lubricate the beds (bottom) of glaciers and cause them to surge, which would mean rapid sea level rise. However, the timescales for glacial surging are not well understood.

The models predict a rise in sea-level between 20 and 80 cm in the next 100 years.

Timescales of climatic response to global warming

CO2 concentration, temperature, and sea level continue to rise long after emissions are reduced

If we eliminated carbon dioxide emissions over the next 200 years, how long would it take for the ocean-atmosphere system to respond?

• The carbon dioxide concentration in the atmosphere would take another 100 to 300 years to stabilize.
• Global temperatures would take a few centuries to level off.
• Sea-level rise would take centuries (for thermal expansion effect) to several millenia (1000's of years (for the ice melting effect).

Other effects of greenhouse gasses

Ocean acidification

CO₂ dissolved into the oceans increases its acidity by producing carbonic acid. This is happening today with the increase in CO₂ produced by the burning of fossil fuels. Higher acidity may have adverse effects on ocean life that produce shells of calcium carbonate (which is dissolved by carbonic acid.)

• A recent article on the topic: http://sciencenow.sciencemag.org/cgi/content/full/2007/217/2 from the AAAS.
• http://www.ipsl.jussieu.fr/~jomce/acidification/

What to do about global warming

Reducing emissions of greenhouse gasses

Sources of greenhouse gas emissions.

Reducing greenhouse gas emissions means reducing the use of fossil fuels. The major sources are:

• Industry, which uses fossil fuels for power. Industry would need to switch to other fuel sources, such as nuclear, hydroelectric or renewable (eg. solar, wind) power, or capture their carbon dioxide emissions before they leave the power plants.
• Transportation: Gasoline is a fossil fuel, so the switch to other sources of fuel or decreasing the gasoline useage of cars by increasing fuel efficiency will reduce emissions. One of the reasons that gasoline in Europe costs twice as much in Europe is because of the gas tax partially designed to reduce gasoline usage.

Carbon sequestration

What if we could capture carbon dioxide before it's emitted, or trap it out of the atmosphere? What would we do with it?

Carbon sequestration means taking carbon dioxide out of the atmosphere (or preventing it from getting into the atmosphere) and storing it somewhere.

Some examples of carbon sequestration:

• Trees and plants: Trees absorb carbon dioxide from the air and use it to produce wood. As a result, growing forests is a method of carbon sequestration.
• Plankton: Small, microscopic plants and animals that live in water bodies are called plankton. Many of these organisms construct shells of calcium carbonate (CaCO₃). So when they die, if their shells sink to the bottom of the ocean and get buried, then they have effectively taken carbon dioxide out of the atmosphere. Experiments have been done where small amounts of iron has been added to ocean water to increase plankton growth and enhance this type of carbon sequestration.
• Oil/gas wells: Oil and gas are extracted from layers of sedimentary rock that are often at great depths beneath the surface. These rocks have been holding oil and gas for millions of years. What if you could pump carbon dioxide into the wells so that it is stored underground? This type of carbon sequestration not only takes carbon dioxide out of the atmosphere, but also can help push more oil and gas out of the wells. For this reason some oil companies are already trying this.

Kyoto protocol

The Kyoto protocal is the international agreement on limiting greenhouse gas emissions. Most of the world's countries are part of the agreement, but the largest emitter of greenhouse gasses, the US, is not. One of the reasons the US has not signed the protocol is that major developing countries such as China and India are not bound under the protocol to make any greenhouse gas reductions.