Why We Know Global Warming is Real

'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. 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.

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 650,000 years (through two ice ages):

Temperature and atmospheric concentrations of greenhouse gases CO₂ and CH₄ derived from air trapped within ice cores taken from ice cores in Antartica. From 2001 Intergovernment Panel on climate change.http://www.grida.no/climate/ipcc_tar/wg1/pdf/TAR-02.PDF

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 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 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, etc.) the closer the model is to the present temperatures. Carbon dioxide and methane (and other gasses) 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.

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. 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 over the history of human civilization (and 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 370 ppm and rising (30% 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 2001 Intergovernment 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.

We can calculate how much CO₂ is going into the air from human activities (burning coal, oil, deforestation, etc.).
Carbon in carbon dioxide can be radio-carbon dated. This tells us if the CO₂ comes from fossil fuels or from other, newer sources (plant respiration for example).
There has been a 31% increase in CO₂ since 1750.
The CO₂ level is 27% higher now than any time in the past 650,000 years (covering four ice ages).
About 3/4 of this increase is due to fossil fuel burning, the rest due to land use changes (deforestation).
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 can absorb is unknown.

Intergovernmental Panel on Climate Change (IPCC): Climate Change 2001: The Scientific Basis (Executive Summary).

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 three cases: 1. No human CO₂ introduced into the model (natural forcing only such as solar changes, orbit changes, etc.). 2. Human CO₂ introduced, natural effects ignored. 3. All effects introduced. 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).

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 greenhouse effect. 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. The effect of ocean warming and ice caps melting will cause sea levels to rise by several meters, inundating costal areas where more than half the worlds population lives and causing flooding disasters such as occurred in New Orleans in 2005.
The ice-core data shows temperature changes of as much as 10C occurring 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. It is not known what the effect of a gradual warming might have on ocean circulation patterns. 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.
Diseases and disease vectors which are now restricted to tropical climates would become more widespread.
Computer models show that the intensity of tropical storms (such as hurricane Katrina that devastated New Orleans) is likely to increase due to warmer temperatures.

From 2001 Intergovernmental Panel on climate change, http://www.ipcc.ch/ (http://www.ipcc.ch/pub/un/syreng/spm.pdf)

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 scientists who disagree with the consensus on global warming. However 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 papers published between 1997 and 2003 out of 900+ papers none, not one, said global warming wasn't human caused).
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 thousands of scientists. 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 (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 30% increase in CO₂ is not normal, that in all the fluctuations over the past 650,000 years we have never seen this amount of CO₂. Yes the earth has been hotter than it is now a few times in the past but the predicted temperature changes we are facing are far more than anything ever seen in 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.
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.