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. Here is a link to my power point on climate change.

The argument in a nutshell:

  1. There is no doubt that we are changing the atmosphere by adding significant amounts of carbon dioxide.
  2. Calculations (based on well understood physical laws and verified with data from the earth, early earth and other planets) show how extra carbon dioxide in the atmosphere affects climate.
  3. A temperature increase (and many other climate effects) has been measured that fits with the calculations.
  4. It matters because a climate change will affect a lot of people, most likely adversely.
  5. Skeptical views are in the minority and supported by the fossil fuel industry.

1. We are doing a huge experiment with our atmosphere.

Prior to the 1900's atmospheric CO2 levels varied from 260 to 290 ppm (parts per million) over the past 800,000 years, including the last 4 ice ages. Due to a century of fossil fuel burning, atmospheric CO2 levels are now above 390 ppm and rising (35% higher than ever before!).

From the Intergovernmental Panel on climate change, http://www.ipcc.ch/ (http://www.ipcc.ch)

We know this extra carbon comes from humans:

2. Scientists can calculate what effect additional carbon dioxide in the atmosphere should have on the climate.

Scientists started thinking about the possibility that carbon dioxide in the atmosphere affects climate more than 150 years ago (see the history of climate change by Spencer Weart). The basic equations governing heat transfer and absorption of sunlight by the atmosphere have been well understood for more than a century. Through a long series of discoveries, calculations and observations of the earth, the other planets in our solar system and data from differ times in earth's history it gradually became clear that CO2 plays a very significant role in climate (both on earth and other planets).

The greenhouse effect.

The greenhouse effect in a nutshell: All objects give off heat mostly in the form of infrared radiation (although hotter objects also give off visible waves). A hot ember in a fire gives off red light but also infrared that we don't see. We can see cooler objects by reflected visible light but they also gives off infrared that we can't see. Night vision technology detects this infrared light even when there is no visible light; The picture below was taken with an infrared camera in a dark room.

The earth receives energy from the sun in the form light that has a maximum output in the visible spectrum and the earth cools due to light that has a maximum in the infrared part of the spectrum (this is called black body radiation). 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 (60F) cooler than what we observe.

Why is the earth warmer than this simple calculation shows it should be? The earth's atmosphere slows the rate at which heat leaves the surface. Oxygen and nitrogen (which make up most of the atmosphere) are transparent to visible and infrared but water vapor and carbon dioxide do not transmit infrared very well. 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 calculation (CO2 levels, H2O levels, convection, evaporation, scattering, reflection, spectral absorption, other gasses, etc.) the closer the model is to the present average temperatures. Carbon dioxide, water vapor and methane (and other gases) keep the earth warmer than it normally would be by about 33C on average (which is a good thing, otherwise we'd freeze to death!). 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. It also has an impact on controlling the water content of the atmosphere which is the other major greenhouse gas. More.

Are the equations right?

We get the right temperatures for earth, are there other tests to see if we have the right math? We can check our equations by applying them to other planets and a few moons of Jupiter. 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 visible and infrared light (how objects are heated and cool) allow us to calculate precisely how much warming there will be due to a given amount of greenhouse gas. The calculation can be done for local regions on earth, for other planets and the moons of Jupiter and we get the right temperatures.

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, Titan, etc.).

Do we have other confirmation that the equations are right?

We can also check the calculations using geological data on earth. 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. Newer (2005) ice core data goes back 800,000 years (through multiple ice ages). The data is global; the same relationship shows up everywhere.

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).

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). Another piece of the evidence linking atmospheric carbon and temperature is the Palaeocene-Eocene thermal maximum. Around 55.8 million years ago there was a sudden increase in atmospheric carbon dioxide and surface temperatures. Although the source of this carbon dioxide is not well understood, the geological record clearly shows a global temperature rise (4C to 6C) which matches the increase in CO2 levels. 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. Calculations of what the temperature should be with these levels of carbon dioxide match the data.

In the past, CO2 and CH4levels have acted as feedback mechanisms, aiding to push the climate into warmer, interglacial periods and also back into ice ages (climate begins to change due to changes in the earth's orbital parameters and CO2 and CH4 levels acted to greatly enhance the small orbital effects). The temperature shift from glacial to interglacial periods is on the order of 7C to 10C (18F!) 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. These shifts are believed to be triggered by orbital changes but require greenhouse gasses to reach the temperatures that are recorded in the data. 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.

What do the calculations say about the future (and are they right)?

Because the calculations are complex they are now done by computers. The figures below show the computer models prediction compared to actual (past) 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, volcanoes, 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 (black body 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 (1.8oF to 6oF) 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 (1oF).

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

3. 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 (below). The maps show where the data came from and this link tells how we got it. 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.

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 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 shifting northwards towards the poles as seen in the image below.

Below is a picture I took of a glacier in Alaska in 2001. 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 Arctic, are subsiding.

4. 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.

5. 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:

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


The data shown here are not opinions, they are scientific facts (i.e. facts accepted by the majority of the scientific community). If you have comments please contact Dr. K. Forinash.