There is a lot of confusion about what is known about global climate change. Some "misunderstandings" are deliberate—-some people stubbornly refuse to believe any fact they wish were not true—-but I will leave the dissection of such political controversies to my esteemed colleagues on this blog. Instead I will concentrate on the scientific misconceptions, many of which are due to the loose use of terms or the conflation of similar but distinct ideas.
1. There have been changes in the composition of Earth’s atmosphere over the past 150 years: TRUE. The most famous example is the 20% rise in the average concentration of carbon dioxide (CO2) in the atmosphere over the past 50 years, as measured hourly (!) by the Mauna Loa observatory. The increase in CO2 concentration has been gradually accelerating over time, and the measurements are confirmed by similar experiments performed at remote locations around the globe, from Alaska to Antarctica. Measurements of CO2 concentrations for earlier centuries are available from ice core samples taken from around the world. Together, the ice cores and the direct atmospheric observations show unambiguously that the increase in CO2 began in the mid-1800s after having been stable for a thousand years. Furthermore, scientists have measured the interglacial fluctuations in CO2 levels over a much longer period and current levels well exceed any previously known level for at least the past 420,000 years.
2. Humans have been responsible for increase in CO2 in Earth’s atmosphere: TRUE. We can measure the amount of emissions of CO2 due to changes in agricultural land use and from burning fossil fuels. Data is available for industrial emissions for over two centuries. The figures are consistent with the observed rise in the atmospheric concentration of CO2. Approximately 1/3 of the increase has been due to changes in land use, and the other 2/3 from fossil fuels.
3. Sufficient increase in the CO2 concentration will cause global warming: TRUE. Every single model of global climate change indicates that this is the case, and we have the planet Venus to look at as an example. Scientists differ in their predictions of how much of an increase is required to trigger catastrophic warming, largely due to the existence of many other influences on atmospheric temperature (including solar output, and perhaps unknown factors), and the sensitivity to positive and negative feedback in the system. So we know that a sufficient increase will cause global climate change, but we are not sure quite how much is sufficient. Even the Republican-led EPA agrees.
4. Global warming has occurred over the past century: TRUE. Yes, even the EPA agrees that we know "for certain" that at least 1 degree (F) of global warming has occurred since the 19th century (they also concur with the man-made increase in CO2). All agree that warming has occurred over both hemispheres, and over land and sea. However, large annual and even decade-long variances are quite normal, as are differences in the trends for sea temperatures, and temperatures in the troposphere vs. the stratosphere. You have to look long-term and broadly to see the full trend—but when you do, the trend is unmistakable. Tree rings, ice core samples, and many other scientific methods confirm that the recent, rapid increase in temperature is significant and unusual, at least since the last Ice Age ended, and possibly for much longer.
5. Humans are responsible for the global warming that has occurred over the past century: LIKELY. We have a smoking gun and can see a dead man lying on the ground, but we cannot speak with certainty; we cannot be sure that the global warming we are seeing is due to human activities. The Intergovernmental Panel on Climate Change (IPCC), whose findings are used as the basis for environmental policy around the world, even by the Bush Administration, said early it was "unlikely" that the warming was due entirely to natural causes, and added in its most recent formal assessment report that it was there was now, "new and stronger evidence that most of the warming observed over the past 50 years is attributable to human activities."
6. Global temperatures will rise in the future: VERY LIKELY. Because it is widely believed that most of the current rise in CO2 is due to human activities, because it is known that a sufficient increase in CO2 will cause global warming eventually, and because the projected increase in CO2 emissions over the next few decades from the growing global economy will bring atmospheric CO2 concentrations to sufficiently dangerous levels as determined by virtually every model of climate change, the IPCC predicts there will be global warming of somewhere in the range of 2.2 to 10 degrees (F) over the next century, if nothing is done. There are many possible uncertainties and possible cooling effects that lead to the broad range of predicted temperature changes, and in some parts of the world, the resulting changes in weather patterns may instead induce local cooling—but the IPCC warns that a global, average increase of even the lowest projected size, "would probably be greater than any seen in the past 10,000 years."
7. Global temperature rise will cause ocean levels to rise, increasing hurricanes, and more violent weather: MAYBE. An increase in ocean levels is a very likely outcome--indeed, there are already measurable increases--but none of the other climate changes are certain, because they require better models of weather than we currently can produce. So in case you were wondering, "The Day After Tomorrow" is an amusing but silly film.
None of this has to do with the ozone hole, increasing methane from cows, decreasing trees in the rainforest, loss of species diversity, nuclear power, El Nino, the Arctic Oscillation, or the vast left-wing conspiracy. It's just good science.