Here are some thoughts on how I interpret the subject of global warming. I think global warming appears to be really occuring and greenhouse gas emissions are likely the majority of the cause. There are some natural variations as well, such as from solar cycles, ocean currents, and volcanic activity.
If we wait until things get obviously worse before we take action it could be too late for reasonably quick action to restore our familiar climate. One aspect of the carbon cycle is that even if the global emission rate is held constant, the CO2 concentration in the atmosphere would continue to rise for quite some time (probably many centuries - until fossil fuels run out) and reach levels several times what it is at present. Alternatively, to hold the CO2 concentration at current levels, the emission rate would have to be cut by more than one-half (as we consider the interactions of the atmosphere-ocean system), and eventually to near zero. To hold the currently elevated temperature constant the emission rate would need nearly a total phase out of CO2. If we magically turned off all emissions at once, some of the CO2 could stay in the atmosphere for a very long time. The rate of decay (based on ocean uptake) would occur on several time scales, so that CO2 levels would come halfway down perhaps in 50 years, then more gradually trails off to reach a new balance between atmosphere and oceans. In the atmosphere a 20% enhancement remains for 10000 years or even more. On this long time scale rock formations ultimately would clean both the atomsphere and oceans of the excess CO2. Carbon sinks other than the ocean would have to be identified to arrive at a more optimistic scenario. One should also consider the challenge of the developing world's desire to play catchup to the level of per capita emissions of the industrialized world when projecting total global emissions.
Summer sea ice in the arctic is thinning noticeably and coverage has decreased significantly by about 25% in the past 30 years. Models and recent trends suggest this ice will probably be mostly gone by the end of the century or even sooner. This suggests a major change to the Arctic climate, albedo and ecosystems. The Greenland ice cap shows signs of melting at an increasing rate and is already contributing about 10% (0.25mm) to the annual rate of global sea level rise. This is in contrast to other studies showing some thickening during the 1990s, so it appears there may be some changes underway in the balance of increased snowfall in the interior compared with accelerated melting along the coast. A total Greenland ice cap melt would correspond to around 7 meters of sea level so the current rate would melt the cap in roughly 36000 years. While this appears to be a comfortable margin it is notable compared with the cap having been around for millions of years. It may also continue the current accelerating trend so this linear melting time could shorten dramatically.
Then again, humans are a part of nature to take the opposing viewpoint, but these disruptions of climate will likely impact the Earth's biodiversity and the balance of ecosystems just as much as inconvenience people.
There is an as yet undetermined risk that the global warming started by human greenhouse gas emissions could start a positive feedback of CO2 and methane release from oceanic clathrates and Arctic permafrost areas. A positive feedback effect is possible if warmer temperatures melt Arctic permafrost thus releasing CO2 stored in peat bogs. As sea ice melts, the lowering albedo of the Arctic Ocean represents another positive feedback. These scenarios could double or triple the currently expected warming and have a more grave effect on human and other lifeforms than is already expected. This possibility should be considered in the "risk assessment" of how aggressively to act in reducing our greenhouse gas emissions. In the past few years methane had been leveling off then started up again, and it remains elevated above background levels enough to contribute about 1/3 of the radiative forcing compared with CO2. We'll have to watch carefully for changes in that trend.
Note with the oceans that as the water warms it would be less able to absorb CO2 keeping more in the atmosphere producing some positive feedback. What CO2 does get absorbed in the oceans makes it more acidic thus threatening marine life. As the oceans circulate they will (as mentioned above) eventually come to a chemical CO2 balance with the atmosphere halting additional uptake. This would set the long term atmospheric CO2 enhancement above background to 20% of the peak value. We then have to wait thousands of years for geological processes to kick in that would incorporate CO2 into rock formations. Other sinks would have to be identified to do what the oceans wouldn't be able to such as forests, soils, or rock formations.
Overall one has to weigh costs vs. benefits in the face of a good if imperfect prediction. How fast would a prudent response be? My rough estimate would be converting to at least 20% renewable (non-polluting) fuels by 2020, 80% by 2050, on up to 100% by 2100. This would still end up giving us several degrees F warming this century, though at least it might be an achievable way out of this conundrum. If we can almost completely phase out greenhouse gas emissions earlier, say by 2060 or so, this could give us a more comfortable margin of safety. This may be necessary in fact unless we can come up with an equivalent strategy that would remove CO2 (i.e. capture/sequestration) that we've emitted into the atmosphere.
It is true that solar forcing could tend to produce an ice age over the next couple of thousand years since the earth's perihelion occurs while the sun shines over the reflective Antarctic ice cap. This may be a mild ice age since the Earth's orbit is more circular these days. However, the amount of human caused global warming appears to be considerably more that what would be needed simply to prevent a future ice age (i.e. too much of a good thing).
Taking a longer view, the current CO2 level (as augmented by fossil fuel burning, etc.) is shown by ice core samples to be the highest in over 800,000 years. Other evidence leads to a best guess that it is now the highest in tens of millions of years. We thus appear to have pushed the earth's atmosphere into a state it hasn't seen since hominids started walking the planet. Taking the really long view, warming will happen in a billion years anyway as the sun consumes its fuel and heats up if we haven't decided to move the earth farther away. It's also interesting to note that in earth's distant past (billions of years ago) we actually needed the much greater CO2 (and methane) amounts that then existed to compensate for the fainter young sun. CO2 was still very high (with a warmer climate) when dinosaurs walked the earth. Higher values in the present era would not be so helpful for currently existing lifeforms.
Steve AlbersP.S. More info in the "Global Warming (& Greenhouse Gases)" section of my weather links , a little past the mid-point of the list. The section just above this called "Ocean Wx" is also useful.