Most of you reading already know I'm an atmospheric scientist, so this topic is near and dear to my heart.
I spend a lot of time hanging out at blogs of so-called "global warming deniers" or "skeptics" to try to understand where they are coming from. It's never accurate to generalize, so it's wrong to lump everyone that is skeptical of human-caused global warming into one pot. It is legitimate to question. I would, in fact, be concerned if people believed anything without questioning and doing their own thinking. However, some of the arguments out there are simply ludicrous (e.g., CO2 is not a greenhouse gas, or human activity is not increasing CO2 in the atmosphere). In fact, these two things are solidly established. CO2 occurs naturally in our atmosphere and absorbs infrared radiation, re-emitting it in all directions (rather than allow infrared heat to escape directly into space), and this ultimately keeps the surface of our planet at the comfortable temperature it is. The average temperature of the earth would be about 60 degrees Fahrenheit cooler without CO2, water vapor, and the other greenhouse gases. It's also easily and well-established that our burning of fossil fuels has significantly increased the amount of CO2 in the atmosphere. We know this from simply looking at the sources and sinks of CO2, including the increase in human sources during the industrial age. We can also can use isotopic analysis to convince us that human activities are responsible for the increase of CO2 from 280 ppm (parts per million) 150 years ago to about 380 ppm (and rising) today. (See, for example, this post from climate scientists at their "Real Climate" website)
Given these two simple facts, the impetus in fact becomes "show me how humans can't be impacting the climate."
Of course it's infinitely more complicated than that. There are climatic feedbacks. We don't fully understand the role of aerosols and clouds. Climate scientists are making much progress at understanding the climate through use of models (only one tool to try to better understand the processes impacted by increased CO2). There is also work afoot to drastically improve the observations that we need to better understand how and why the climate is changing. There are a multitude of signals that we're monitoring to follow the impact of climate change: e.g, surface temperature (from surface stations), near-surface temperature (from satellites), polar ice melt during the spring and summer, glacial melting, average sea level, ocean temperatures, changes in permafrost, migratory behavior of species...
There is overwhelming scientific consensus that human activities are impacting the climate. The areas of active research - the areas where there is not consensus - are focused on the expected magnitude of the changes, and how increased CO2 and the accompanying average warming might impact things like agriculture, species migration, and regional climate.
I thought I'd also point out that the idea of global warming is not as recent as you might think. Here's a brief history of some of the early key players.
John Tyndall was a British physicist, born in 1820, who loved mountaineering. He adored nature, and in particular loved geology.He was fascinated by the idea that geologic evidence showed that northern Europe was once covered with ice. He was amazed and puzzled that the earth's climate could change so significantly. This curiosity led him to examine the ideas put forth about 30 years earlier that some of the gases in the atmosphere might in fact, be capable of absorbing and reemitting heat energy (infrared radiation).
So in 1859, John Tyndall published results from laboratory studies that in fact did show that several gases that were commonly present in the atmosphere were absorbers of heat. (Check out that link - it's a copy of his 1859 paper.) Water vapor and carbon dioxide were two of the most important gases with this property.
Here's his lab set-up. (Much of this information was obtained here). On the far left is a bunsen burner (sitting atop the desk). It is heating a Leslie's cube which is filled with water, and there is a thermometer sticking out of the top to measure the heat of the water. It's hard to make out, but there's another Leslie's cube being heated by a bunsen burner at the far right side of the long horizontal tube. The long tube is filled with the gas of interest. Tyndall ran experiments on several different gases. The funny looking thing with two cones on it (sitting on the desk between the long tube and the first Leslie's cube) is a thermopile. This instrument generates electricity when there is a heat differential – that is, when one side is warmer than the other. It's connected to a galvanometer to measure the electric current that is generated. As the infrared energy from the 2nd Leslie's cube passes through the gas in the long tube, if the gas is an absorber of the infrared radiation, the temperature at the end of the tube will increase, resulting in a temperature differential that can be measured by the generated electric current. What John Tyndall discovered was that water vapor (aqueous vapour), carbon dioxide (carbonic acid) and ozone were particularly strong absorbers of infrared radiation while being transparent to visible light. Here's some of what he wrote:
"But this aqueous vapour, which exercises such a destructive action on the obscure rays, is comparatively transparent to the rays of light. Hence the differential action, as regards the heat coming from the sun to the earth and that radiated from the earth into space, is vastly augmented by the aqueous vapour of the atmosphere."
"...Now if, as the above experiments indicate, the chief influence be exercised by the aqueous vapour, every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid diffused through the air, while an almost inappreciable admixture of any of the hydrocarbon vapours would produce great effects on the terrestrial rays and produce corresponding changes of climate."
"...Such changes in face may have produced all the mutations of climate which the researches of geologists reveal."
About 30 years earlier, Joseph Fourier, who was a French mathematician and physicist (and yes, this is the same Fourier of the mathematical Fourier Series), had published work where he noted that the temperature at the surface of the earth was much warmer than you would expect from doing a simple energy balance of the planet.
"The decrease of heat in the higher regions of the air does not cease, and the temperature can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, then in repassing into the air when converted into non-luminous heat."
His theory was that the atmosphere must allow visible light through easily, but infrared heat energy must be absorbed by the atmosphere. (He also had some idea that the earth received heat from "the universe at large" but that's since been disproven, heh.)
Building on these studies, in 1896 a Swedist chemist names Svante Arrhenius refined the idea of the greenhouse gases. He suggested that halving the amount of CO2 in the atmosphere could decrease the global temperature enough to explain previous ice ages. He also worked with a colleague Arvid Hogbom and showed that the amount of CO2 put into the atmosphere by burning fossil fuels was roughly equivalent to natural processes. He postulated that given enough years, the amount of CO2 might build up enough to begin to impact the climate. This was in 1896. Arrhenius and Hogbom figured it would take about 3000 years to build up enough CO2 to see an impact on temperature, given the emission rates of 1896. We started burning fossil fuels considerably faster than the rate of 1896. Don't you wonder what these scientists would think of how things have played out today?