12/14/2009 04:45 pm ET | Updated May 25, 2011

Copenhagen Climate Primer

Did you hear? The Copenhagen talks have been suspended. Another walkout. The disagreement is over policy, not science (and some call the move tactical and a possible sign of progress). But since there's been a kind of walkout over here in public opinion, here's some basics on the science to keep in mind for when folks walk back in.

Amid all the debate about e-mails between scientists and emissions from China, let's not lose sight of the basics.

The evidence of global warming and the role of human activities in that warming is strong. And it goes well beyond whether temperatures over the past 1,000 years are shaped like a hockey stick or whether climate models can accurately reproduce climatic variations.

As in all areas of science, the proof is in the data from observations.

Global Temperatures Are on the Rise

Our record of global temperatures from direct measurements goes back about 150 years. It shows a clear, long-term upward climb in global temperatures -- today's average global temperature is some 0.9 degrees Fahrenheit above that of 1880. And despite all the claims that global warming has stopped since 1998, it appears that the period from 2000 to 2009 will prove to be the warmest decade on record.

Some have challenged this global temperature record. Despite the best efforts of scientists to filter out confounding effects such as urban heat islands, many skeptics continue to challenge the trends derived from the record.

Fine, you don't have to be a statistical maven of global temperature measurements to appreciate that the globe is warming. Glaciers and sea ice the world over are melting. Case closed.

A Role for Humans

All too often scientists explain the evidence of human interference in climate change using results from climate-model simulations. This explanatory technique is unfortunate because these models, while highly sophisticated, are in the end just numerical representations of the way we think our planet works. And the evidence of human interference is a lot more basic than model simulations -- as basic as the most fundamental law in physics, the conservation of energy.

Conservation of energy demands that any increase in temperature for any body including the atmosphere must be accompanied by a source of extra energy or heat. If atmospheric temperatures are rising, if glaciers are melting, excess heat must be coming from somewhere. This applies to all possible causes of global warming. Natural causes of climate change, so-called internal oscillations, manmade climate change -- nothing gets a pass from the law of the conservation of energy.

So, what is the source of heat causing global warming? It turns out that scientists have been asking themselves that question for decades and have been using advanced instruments and satellites to get answers. And here are the answers:

  • It's not the Sun. Satellite data show that over the past ~ 30 years, there has been no net change in the total solar irradiance (the amount of energy emitted by the Sun). The same is true of cosmic rays and the solar wind.
  • Not heat from the ocean. For the ocean to have supplied the heat needed to cause global warming, the heat content of the ocean must have decreased. Why? Conservation of energy: if the ocean is supplying heat to the atmosphere, its own heat content must be decreasing. But measurements data show that, if anything, the heat content of the ocean has increased while the atmosphere's heat content has increased. It seems that whatever is driving an increase in atmospheric temperatures is also warming the oceans.
  • Not reflection. Much has been made of clouds as a possible driver of global warming. Changing cloudiness can have a huge impact on the climate by changing the Earth's reflectivity or albedo. Other processes such as changing land-surface characteristics or changing the atmospheric load of scattering aerosols can also affect albedo.

If the Earth's albedo has been slowly decreasing, then it would cause the planet to absorb extra energy from the Sun and explain global warming. But here again we have direct observations that appear to rule out this possibility. The Earth's albedo has at times increased and other times decreased, but has not steadily decreased in the way needed to explain global warming.

  • Greenhouse gases supply the heat. Unlike the other putative sources of global warming, greenhouse gases and specifically carbon dioxide (CO2) have been observed to be heating the atmosphere.
  • Satellites looking down on the Earth detect a hole or dip in the infrared radiation emitted by the Earth and sent into the space at exactly that part of the spectrum that corresponds to where CO2 absorbs radiation. Since the Earth emits infrared radiation to balance the absorption of energy from the Sun, blocking the emission of this radiation acts effectively as a source of heat to the planet and leads to global warming. That this occurs is not a theory; it is directly observable from space.

    So, by the process of elimination, we are left with only one viable explanation for global warming -- greenhouse gas pollution. To advance any other explanation requires that a source of heat be identified and thus far, as hard as scientists have tried, no such source has been found.

    Let me repeat:

    • The evidence that humans are causing global warming is not a theory.
    • It is not based on a model simulation.
    • It is founded in the law of conservation of energy and direct observations of the Earth.

    What About the Future

    Ah, you say, maybe humans are causing global warming, but how do we know what will happen in the future? If you agree that models are not all that trustworthy and models provide the only way to predict the future, then you can't be sure how the climate will change in the future if we continue to emit greenhouse gases.

    And I agree: predicting the future is a bear. We really don't know what the climate will do in the coming decades.

    But don't forget that uncertainty cuts both ways. Maybe the models have overestimated the effects of greenhouse gas pollution, and maybe they have underestimated the effects. With every extra megaton of carbon dioxide we add to the atmosphere, the chances of serious adverse consequences increase.

    And there's the rub for the folks in Copenhagen. Come to a global agreement about greenhouse gas emissions or place your bets, ladies and gentlemen, on the well-being of future generations.

    Originally posted at