What We Know About Climate Change

'The climate system is a capricious beast, and we have been poking it with a sharp stick' W.S. Broecker

A brief explanation of the data supporting the idea the humans are altering the environment and why this is a bad thing.

1. There is no doubt that earth surface temperatures have been rising sharply since the beginning of the industrial revolution.

The (newest and improved) "Hockey Stick" diagram (above). The maps show where the data came from. Each colored line represents different studies conducted by different researchers, all converge on the same disturbing conclusion: Our planet is warming up and the trend appears to be increasing over the last century. Gavin Schmidt, Michael Mann, and Stefan Rahmstorf from REALCLIMATE.

NOTE: This isn't the only evidence of climate change. Changes in ice coverage over Greenland and other polar regions have been documented both on the ground and with satellite data and, because the poles are react faster to climate change, are significantly larger than for middle latitudes (15% to 20% decrease since 1978). The level of the oceans is rising faster now than in the past (2.7 inches in the past 40 years). Other supporting data include the more than 28,000 cases of changes in biological marine and land systems which have been documented including shifts in migration patterns, reproduction times, leaf maturation and plant blooming times. Plant and animal habitats (hardiness zones) are migrating towards the poles at a faster rate.

The above is a picture I took of a glacier in Alaska. The sign shows where the face of the glacier was in 1951. The current face is the blue ice in the background two miles away (you may barely be able to see people standing in front of the glacier in the picture- they appear as small specks). Nearly all the Glaciers around the world, including Antarctica and the Artic, are subsiding.

2. We know CO₂, carbon dioxide, is closely linked with temperature in the earth's past.

Both ice core data and ocean sediment data going back 400,000 years show a direct correlation between temperature and CO₂ levels. New (2005) ice core data goes back 800,000 years (through multiple ice ages).

Temperature and atmospheric concentrations of greenhouse gases CO₂ and CH₄ derived from air trapped within ice cores taken from ice cores in Antarctica. More data at: Intergovernmental Panel on Climate Change (http://www.ipcc.ch/ipccreports/index.htm).

Data for times before 800,000 years ago are harder to find because ice and ocean sediment records are incomplete. But there is at least some evidence that 10 million years ago all ice was melted, sea levels were much higher and CO₂ levels were higher (400ppb). There is other evidence that the earth had a warm climate 1.4 billion years ago in spite of a much weaker sun because of high carbon dioxide levels in the atmosphere.

In the past CO₂ and CH₄ levels have acted as feedback mechanisms, aiding to push the climate into warmer, interglacial periods and also back into ice ages. It should be mentioned that the temperature shift from glacial to interglacial periods is on the order of 7C to 10C and occurs very rapidly, in as little as a few decades. Shifts from interglacial periods back to ice age are not as rapid but are sill relatively fast. The figure also shows the very recent sharp increase in CO₂ and CH₄ and N₂O levels as measured directly from the atmosphere. Approximately 40% of the N₂O , another green house gas, in today's atmosphere is the result of human activity including crop cultivation (especially where nitrogen is used as a fertilizer), biomass burning and industrial process such as nylon production. As a final note, the current interglacial period of the past 11,000 years has seen relatively constant temperatures compared to other interglacial periods where temperatures varied quite a bit. It is this long period of stable climate that allowed human civilization to develop

3. We know the 'greenhouse effect' changes the earth's surface temperature in a significant way.

The greenhouse effect in a nutshell: All objects above absolute zero Kelvin (-273C) give off heat in the form of blackbody radiation (a broad range of electromagnetic waves which may include visible light and infrared among others). A hot ember in a fire gives off red light as part of its blackbody spectrum. A human body gives off infrared light (not visible to the human eye but used in night vision devices) as part of its blackbody spectra. The earth receives energy from the sun in the form of blackbody radiation (which has a maximum output in the visible spectrum) and the earth cools due to blackbody radiation (which has a maximum in the infrared part of the spectrum). If we calculate the surface temperature of the earth as a result of this heating from the sun and cooling of the earth we come up with a temperature which on average is 33C degrees cooler than what we observe.

Why is the earth warmer than calculations show it should be? The earths atmosphere slows the rate at which heat leaves the surface. If we do this calculation again taking into account the effect of the atmosphere which decreases the rate that energy leaves the earth in the infrared we come up with the correct average surface temperature. A simple, back of the envelope calculation comes close to the right answer and the more details we put into our model (CO₂ levels, H₂O levels, convection, evaporation, etc.) the closer the model is to the present temperatures. Carbon dioxide and methane (and other gases) keep the earth warmer than it normally would be by about 33C on average (which is a good thing!). A similar phenomena affects the surface temperature of Venus, Mars and Titan; in particular Venus' surface temperature is approximately 400C degrees warmer than it would be without its atmosphere as a result of the greenhouse effect. Our theories about blackbody radiation (heating and cooling of objects) allow us to calculate precisely how much warming there will be due to a given amount of greenhouse gas. Although CO₂ makes up a very small percentage of the atmosphere it has a big effect because it absorbs radiation at exactly the frequencies that the earth uses to release heat absorbed from the sun. More.

There are many phenomena which affect the earth's temperature (variations in the earth's orbit, variation of the intensity of the sun, sunspots, various feedback mechanisms, etc.) but calculations which include these effects show that the 'greenhouse effect' definitely plays the largest role in determining the surface temperature of the earth (and Venus, Mars, etc.).

4. There is no reasonable scientific doubt that CO₂ levels have risen sharply since the beginning of the Industrial Revolution.

Prior to the 1900's atmospheric CO₂ levels varied from 260 to 290 ppm (parts per million) over the past 650,000 years, including the last 4 ice ages. Due to a century of fossil fuel burning, atmospheric CO₂ levels are now above 380 ppm and rising (35% higher than ever before!). Global average temperatures are rising in lock step with this increase in CO₂ levels (see the previous two graphs). Note how the green (nitrogen dioxide), red (carbon dioxide) and blue (methane) curves shoot way up on the right side of the graph.

From Intergovernmental Panel on climate change, http://www.ipcc.ch/ (http://www.ipcc.ch/pub/un/syreng/spm.pdf)

5. Approximately 30% of the CO₂ in the atmosphere today can be attributed to human causes.

Carbon in carbon dioxide can be radio-carbon dated. This tells us that the CO₂ comes from fossil fuels, not from other, newer sources (plant respiration for example).
85% of the world's energy comes from fossil fuels. When we add up all the carbon released from burning this fuel over the past 250 years and account for land use changes the number we get is slightly less than the 35% extra CO₂ measured.
The amount of CO₂ in the atmosphere is actually a bit less than what is predicted from our carbon use. We now know that a significant amount of CO₂ is going into the ocean. How much more CO₂ the ocean will absorb is unknown.
There has been a 35% increase in CO₂ since 1750 which is the beginning of the heavy use of fossil fuels.
The CO₂ level is 35% higher now than any time in the past 650,000 years (covering four ice ages). Methane is 150% higher.
Methane comes from bacteria in rice fields, digestive track of cattle, fossil fuels, termite colonies, sewage treatment plants and land fills. All except termite colonies are human related and are increasing with the world population.

6. Computer models are able to accurately predict past temperature changes and predict a future rise in temperature.

The above figures show the computer models prediction compared to actual observations in two cases: a) Human CO₂ and other human effects included. b) No human CO₂ introduced into the model (natural forcing only such as solar changes, orbit changes, etc.). Obviously only the model with human contribution matches the actual data. This tells us that our computer models are accurate and include the important sources of temperature change on the earth's surface (natural and human). This shouldn't be surprising, the models are not guesses nor are they trend curves (like is done in business projections); they are calculations based on the same physical laws that we use in other scientific applications (blackbody radiation, absorption and re-emission of electromagnetic waves by water, methane and carbon dioxide, convection of the atmosphere, fluctuations in solar radiation, etc.).

These same models (which work so well for past data), when run forward into the future predict an increase in global temperature between 1C and 3C (1.8^oF to 6^oF) in the next 100 years, depending on how much we modify CO₂ output levels (the higher numbers for no modifications of current trends). If we halt CO₂ levels at the values they were in 2000 the models still show an increase of around half a degree Celsius (1^oF).

The curves after 2000 represent various models with different assumptions about how human activity might change in the future.

7. Why should we care?

Some people have argued that the world could adjust to a general warming of the climate or even that warming would be a good thing. This would be a very dangerous experiment to undertake.

If CO₂ emissions are reduced and atmospheric concentrations stabilize, surface air temperature will still continue to rise slowly for a century or more due to the CO₂ already there. If we wait until we begin to see bad effects we will not be able to reverse them for a century or more.
Thermal expansion of the ocean continues long after emissions have been reduced, and melting of ice sheets continues to contribute to sea-level rise for many centuries. Oceans have already risen by 22cm in the past century. The effect of ocean warming and ice caps melting will cause sea levels to rise by several meters, inundating costal areas and causing flooding disasters such as occurred in New Orleans in 2005.
The ice-core data shows temperature changes of as much as 10C (18F) can occur in as short a period of time as 5 to 10 years. Such a rapid climate change would be catastrophic to agriculture world wide as well as natural species. These rapid changes are thought to occur as a result of sudden changes of the ocean circulation patterns and the release of methane frozen into the continental shelfs. It is not known what the effect of a gradual warming might have on ocean circulation patterns and frozen methane in the sea floor. From: 'Climate Shock: Abrupt Changes over Millennial Time Scales' by E. Bard Physics Today vol. 55 No. 12 December (2002) p32 and 'The Discovery of Rapid Climate Change' by S. Weart Physics Today vol. 56 No. 8 August (2003) p30.
Ice sheets may melt much faster than previously thought to relatively small increases in temperature. Fluctuations of sea level due to ice melt have been as much as 9 meters (enough to cover Florida) for temperature fluctuations of only a few degrees Celcius during the last interglacial period. (About a billion people in the world live within two meters of sea level.)

There are serveral cases in the fossil records of sudden mass extinctions of animal and plant species associated with increases in temperatures. Global warming may trigger such an extinction event if it occurs faster than animals and plants can adapt.

Additional carbon dioxide in the oceans will destroy coral reefs which support many fish species (including those the world depends on for food).
Diseases and disease vectors which are now restricted to tropical climates would become more widespread.
The intensity of tropical storms (such as hurricane Katrina that devastated New Orleans) is likely to increase due to warmer surface water temperatures. Extreme weather events (draughts, floods and tropical storms) are all predicted to get more intense.

8. What about the skeptics?

There have been a few people to come out against global warming saying either it is a normal (not man made) process or is not important. Fox news has been very diligent in giving these people airtime. A couple things to keep in mind:

It is the business of news reporters to find contrary opinions (it isn't interesting news unless it is controversial). There are even a few scientists who disagree with the consensus on global warming (however most of the deniers are not scientists working in climate). These scientists constitute less that one percent of the scientists working in the field. The vast majority of the scientists who have examined the data are convinced that the earth is warming in an unnatural way due to human activity (in one survey of peer reviewed scientific papers on climate published between 1997 and 2003 out of 900+ papers none, not one, said global warming wasn't human caused).
Follow the money. IPCC scientists are not paid a salary, only travel and living expenses while at the meeting. They would most likely recieve more money for further research if this issue was not settled but the majority say it is settled. Most anti-climate spokespersons and lobby groups receive money from the fossil fuel industry. They benefit from postponing any action which might restrict the use of fossil fuels so it is in their interest to confuse the issue. A site that tracks the political/money conections to climate issues is DeSmogBlog.com.
Popular press does not often present hard scientific data but rather presents individuals (people are more interesting than graphs). But global warming is not just the opinion of a few experts, there is a huge amount of global warming data representing millions of hours of work behind those talking heads. A quote by a scientist on a news program may represent the view of thousands of published papers (or in the case of a skeptic, only a handful). You usually can't tell simply by looking at the news program who represents the majority view and how strongly the majority view holds that opinion.
Popular press represents opinion, not consensus. The scientific world operates on consensus (ideas are debated freely but they don't go into the textbooks until everyone agrees they are correct or at least the best information we have). The peer review process of the IPCC is far more stringent than reviews for papers published in most scientific journals; the ideas published there represent the consensus of more than 2000 scientists from 130 countries. An op-ed piece in the paper or on TV represents one persons opinion.
Counter arguments to the causes of global warming are often presented in the popular press which have been examined and rejected by the scientific community. You hear reporters saying that perhaps changes in the solar cycle are the sole cause of climate change, not humans. But the effect of these solar fluctuations have been calculated and included in all of the computer modeling and temperature projections for years now (these fluctuations exist but their effects are overwhelmed by CO₂ effects). You hear people saying that temperature on the earth changes in a cyclic way (it does) and so the current changes are part of the normal ice age cycle. But scientists are saying exactly the opposite, that the current 35% increase in CO₂ is not normal, that in all the fluctuations over the past 800,000 years we have never seen this amount of CO₂. Yes the earth has been hotter than it is now a few times in the distant past but the predicted temperature changes we are facing are human caused and not part of the normal ice age cycle.
Some people seem to be afraid that increased government regulations on carbon emissions would hurt the economy. There is historical evidence contradiction this idea. Many energy regulations went into place in the 1970s after the first oil crisis. This actually spurred the economy at the same time energy consumption remained level for 10 years. The fossil fuel industry will be hit, yes, but the econcomy will grow in other areas like batteries, alternative energy, etc.
If you are still skeptical and think you have a point against global warming that has not been answered here try the much more comprehensive site "How to talk to a climate skeptic" the Real Climate site is also informative as is the Intergovernmental Panel on Climate Change (IPCC) site.