For more in-depth information there are many places to go (the Wikipedia articles, for one, are not bad). A good book is The Science and Politics of Global Climate Change. A good blog discussing the latest scientific developments is RealClimate.org. Professional societies such as the American Geophysical Union and the American Meteorological Society have issued position statements supporting the science linking climate change to human activities. Some common misconceptions and misinformations about climate change are refuted very nicely by the New Scientist.
No question.Every global indicator shows warming: three independent instrumental networks, sea ice (retreating), permafrost and glaciers (melting), species migrations (poleward), borehole and tree-ring-derived temperatures (upward), and spring thaws (earlier)....and don't forget bathing suits
The warming has varied regionally, but the global average temperature now is about 1.2 F warmer than 100 years ago and probably the warmest in at least a millenium [NRC 2006]. Europe and Asia have warmed more than the US, and the Arctic the most, enough to significantly affect indigenous peoples and wildlife.
The media love to dwell on it, making it seem worse than it is. No one event can be blamed on any particular culprit. But some aspects of the weather (see below) are getting a boost from climate change.
Attribution of increased tornadoes, disease, and insurance payouts to recent climate change is probably wrong (cooler heads think the true causes are, respectively, improving detection, increasing population density and air travel, and more property at risk). But other impacts are real, and I expect more serious ones are lurking over the horizon. As for the Nobel, well, depends on the competition! Update 10/07: he won! Congratulations, vice president Gore.
Most agree that warming must—other things being equal—slightly increase the maximum hurricane strengths, but that comes nowhere near explaining the recent near-doubling of Atlantic hurricane activity. Some of my colleagues argue that global warming is responsible, but the evidence is ambiguous and it's unexplainable by current theory, so I wouldn't sell your gulf coast property just yet. My guess is it'll be many more hurricane seasons before this is settled. [Update 6/08]: Two recent studies have cast further doubt on whether hurricane intensity or frequency increases will be all that severe.
It was successfully addressed through an international agreement to phase out freon-type gases once used in refrigerators, air conditioners and spray cans.
We are rapidly learning more about past changes from examination of geologic evidence, which is probably the most exciting field in climate science today. Some changes have been huge: 55 million years ago the north pole was as warm as Florida today, while 20 thousand years ago, much of North America was under miles of ice. The really warm climates all occurred long before the advent of upright primates, but prehistoric humans made it through some pretty cold times. Since the dawn of agriculture, climate has been relativey boring.
Science is a process of testing hypotheses against evidence. Many scientists, using different methods, have tested the hypothesis that the recent warming is also a natural shift. Every analysis has yielded the same answer— "no"—the warming since 1970 has been too widespread and too rapid. A few scientists still reserve judgment on this, but no model has been able to explain the warming naturally. Mountain ice and polar ice shelves that are thousands of years old are melting, which would be an amazing coincidence if humans weren't involved.
Scientists have predicted for over 100 years (see a good history of the science here) that this would warm the planet by adding to the atmosphere carbon dioxide, a so-called "greenhouse gas" that absorbs infrared radiation, inhibiting planetary cooling. Greenhouse gases are naturally present, and without them we'd all freeze, but we've added a lot more. Their importance in influencing a planet's climate is supported by Earth's geologic record and by comparing the climates of Earth, Mars and Venus. The only question is exactly how sensitive our climate is to them.
We try to calculate this from known physical laws. Over the last four decades these calculations have grown pretty complicated, predicting more details like cloud cover and type for example. That hasn't changed the answers much. Every calculation has predicted a substantial warming (generally betweeen 3 and 8 F globally) if atmospheric carbon dioxide is doubled from 0.028% to 0.056% of the atmosphere by volume. It's currently 0.038% and climbing fast. Human activities are also adding other greenhouse gases like methane and nitrous oxide, but carbon dioxide is the most important.
Natural factors like volcanic eruptions, shifts in ocean currents, and fluctuations in solar luminosity have been important in driving small climate changes over the past few millenia, but have not increased in recent decades. Human air pollution (the kind that causes acid rain) is also important, and counteracted part of the greenhouse warming during the 20th century, but is doing so less now due to pollution controls (and would be unable to keep up even if we went back to the bad old, dirty days). Deforestation and agriculture also affect local climate as well as releasing additional greenhouse gases.
The key thing is that we are hitting the climate system harder than any known natural forces. That's true regardless of uncertainties in how the climate system responds. And the recent warming is right in line with longstanding predictions (for example, has since tracked the predictions published by the IPCC in 1990).
We now have very accurate histories showing little change for millenia, then a meteoric rise after coal came into wide use in the early 1800's. Carbon-14 measurements also confirm that the added gas comes from ancient fossil fuels. Other greenhouse gases like methane followed a similar trajectory, though their sources are more complicated.
CO2 is poisonous to humans, but not at concentrations that we could ever create globally.
It forms carbonic acid when dissoved in water, which is making the oceans more acidic. This will eventually harm corals and other organisms at the base of the ocean food chain because their shells will dissolve. This consequence may be as serious as climate change, but isn't getting the same attention, in part because it is less controversial! (did you realize the more agreement there is among experts, the less you'll hear about it?)
Finally, carbon dioxide helps plants grow, but not necessarily the ones we want (one of the strongest beneficiaries is evidently poison ivy! This and other vines are taking over some forests). Also, the extra growth comes at the expense of lower nutritive value, so it isn't clear that this will be helpful from a food point of view.
Studies of "carbon fertilization" of plants (the potential increase of a given plant's uptake of CO2 when there is more in the air) have shown a limited impact over time, so to increase plant uptake significantly would require more plants. Widespread forest planting would help, but only enough to delay warming by maybe a decade or so, and only if the land remains unneeded for other uses. Unfortunately, future warming will probably work against us by causing soils to start releasing stored CO2.
1) This is only the tip of the iceberg. If we burn through most of Earth's fossil fuels we could quadruple atmospheric carbon dioxide, causing an estimated global mean temperature rise of 6-16F. We could easily do this in under 200 years even if energy consumption stopped increasing. Also, even if we ceased all burning immediately, another degree F or so of warming is "in the pipeline" from past emissions.
2) It isn't just the heat, it's the humidity! A 6F warming causes about a 25% increase in absolute humidity. This would make each summer heat wave feel 10-14F hotter (to a warm-blooded creature which I assume includes most readers).
3) Most of life lives, in various ways, "on the edge." 1 F is enough to bleach corals, for example. A tree, animal, or skier has to move about 100 miles poleward for each 1 F of global warming to maintain the same climate envelope. At current warming rates this is several miles per year, which is impossible for most plant species, so forests will get left in the dust.
4) Other things besides temperature and humidity will be affected (see next question).
Obviously we'll have milder winters and hotter summers. In addition we have good reason to expect:
Impact severity is expected to scale with the square of the amount of warming. In other words, a little climate change isn't so bad, but a lot is. Cooling wouldn't be good either; all life on Earth is finely tuned to the current climate.
All reputable scientists, even outspoken "skeptics" acknowledge that we have caused greenhouse warming and it will continue. Uncertainty or dispute are limited to the severity of future changes, and the extent to which certain recent trends (like the loss of the snows of Kilimanjaro) are attributable to greenhouse gases. These are obviously important matters, but not the same as disagreeing about the fundamental nature of the problem. There are occasionally scientists who venture outside their area of expertise, and a few quacks who will dispute even the basic points, in public or before Congress, but usually not among scientists because of the ridicule that would follow. Unfortunately some groups invested in the status quo have run disinformation campaigns to confuse the public. Disagreements are highlighted by the media, while concensus is too dull to report.
A few of my colleagues claim that model predictions of future warming are excessive. They have no calculations to back this up, and in my view their claims have no valid scientific foundation—though they can't be proven wrong per se until warming is fully realized.
Past contrarians whose ideas were initially rejected but turned out to be right, like Alfred Wegener and others who proposed plate tectonics, typically had a new theory that explained data better than the prevailing wisdom. They were often outsiders. Their new theories made the reigning scientific "chiefs" look bad and challenged deeply ingrained notions, so only when the chiefs died off did the new theory enter the mainstream. That description does not fit today's climate "contrarians": they have no new theory, but only criticize well-tested existing theory (and win lots of attention). Support for their views has steadily eroded (in the 1960's most scientists were skeptical about any theory on climate) and is essentially nonexistent among the youngest generation of scientists.
Each individual scientist must compete for funding. We play up the importance of our own speciality: (imagine arrogant nasally voice) "what's holding up climate science is that we don't understand [fill in the blank], so fund my research!". This is more justifiable in some cases than others, but the net effect of all of us doing it is to make it sound like we hardly know anything! Also, any scientist who comes up with a convincing challenge to the prevailing view gets a big career boost—it doesn't pay to jump on bandwagons, especially if they're headed in the wrong direction.
There was definitely no documented "consensus" on any theory of climate, analogous to that articulated today, prior to the 1970's despite what some will imply as they hold up "global cooling" articles from old issues of Life or Readers' Digest.
Atmospheric scientists have always known that fossil fuel burning would eventually cause warming, though the relative importance of this emerged gradually (see Spencer Weart's excellent history of the science). Meanwhile geologists learned in the 20th century that the ice ages were cyclical, so another should arrive. The fact that this would take many thousands of years was lost in the resulting media scare.
Quantifying the economic impact of climate change if we don't change course is quite difficult and estimates (more uncharitably, guesses) range considerably. Some economists say don't worry. My guess is that we will be surprised and that these surprises could be far, far nastier than a few percent loss of GDP--or not.
"ask yourself one question: 'Do I feel lucky?'"--Dirty Harry
The US, China and India are gearing up to rapidly expand their electricity generation and refining capacity, and they are currently planning to stick with old, polluting 20th-century technology even though others are available. This would be a disaster, committing us to a huge, additional belch of greenhouse gases over the next few decades that would more than double our previous emissions and guarantee more serious impacts down the road. Now is a pivotal time in world history.
The point is that we are already "acting" by building more stuff. And since we'll never know exactly what climate change will bring until it's far too late, waiting for "better science" is a de facto decision to give up.
Technological progress is the key but only happens when spurred by markets or government mandates. Meanwhile, gases are building up relatively rapidly, and removing them from the atmosphere (though technically feasible) will probably remain impractical. Geoengineering to counteract climate change could help, but might not go as planned, would have to be maintained by our descendents for centuries, and would not address the the non-climatic effects of CO2. There are a few "no regrets" actions that would have economic or health benefits even in the absence of a climate problem (like eliminating subsidies that now prejudice us toward fossil fuel use, or pursuing more sensible coastal development policies). These would at least get us started.
Ultimately, to achieve meaningful reductions in emissions would require some combination of wholesale shifts to nuclear power, massive increases in energy efficiency, or capture and storage of carbon dioxide before it reaches the atmosphere (check this out—it may be our best option).
The "green" impulse is to conserve energy. Unfortunately this will do little by itself to improve climate. It does however have two very important, indirect benefits:
"The time to repair a roof is when the sun is shining."--John F. Kennedy
Since the last congressional elections, politicians are suddenly talking about this issue (although it not competing well with talk about high gasoline prices). Listen carefully to their proposed solutions. Subsidizing new technologies is popular because it doesn't involve any tangible sacrifice. This may be helpful, depending on what is being subsidized. But plenty of energy-saving technologies have already been developed (many after the 1970's oil shocks) and have been gathering dust on the shelf. The market isn't demanding them because consumers are not individually rewarded for adopting them or just using less fossil fuel.What you and I can do:
