Overview of Global Climate Change
In February 2007, the fourth IPCC report was issued. The IPCC Consensus process is conservative by nature and many divergent opinions are not well represented in the report:
The consensus summary statement of this report, however, is the most strongly worded one to date (although it still is fairly whimpy):
AT4: 2007: Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the increase in anthropogenic greenhouse gas concentrations
However, in order to appreciate the complexity of understanding the problem, let's review the evolution in our understanding of what constitutes a climate system and what the relevant feedbacks are.
In the mid 1970's, our conception of a climate system was naively simple:
By the mid 1980's, it was recognized that the surface reflection and absorption properties greatly influenced the overall energy budget and flow through the Earth's atmosphere, and therefore surface reflectivity was incorporated into our overall model of climate:
By the time of the first IPCC report (1990), the role of the ocean as a gigantic "swamp" which was a temporarily holding place for CO2 was strongly recognized. The CO2 exchange rate between the ocean surface and the atmosphere became an important parameter in our understanding of climate.
By the time of the second report (1995), it became clear (partly due to the
Pinatubo eruption of 1991) that the role of aerosols (sulfate pollution) was
critical. Sulfates, emitted by most industrial processes, rapidly rise through
the lower portion of the atmosphere (because they are very light) and reach the
stratosphere. The stratosphere is an unmixed part of our atmosphere and once
particles reach it, they tend to stay there for decades. Sulfate aerosols
increase the amount that incoming sunlight is scattered 
the main result is that less sunlight
reaches the surface of the Earth, and this leads to a net cooling.
For the first time, it is now realized that industrial processes can produce pollution that either cools the surface or warms the surface. So now, we are in a tug of war between heating and cooling. Recently this cooling by aerosols has been given the media term global dimming.
More complexity arrived with the Third Assessment Report (TAR) issued in 2001. The role of plant absorption of carbon was directly incorporated in to the carbon budget but, more importantly, the recognition that deep ocean transport of carbon might be the most important parameter in the ocean's overall ability to absorb our increasing amounts of carbon emission. Unfortunately, as we will see later, the oceans are now becoming saturated so that our continued carbon emissions now more readily accumulate in the Earth's atmosphere.
Finally, in the fourth report various forms of atmospheric chemistry and changing vegetative patterns were not part of the overall climate model and the assessment of the state of the climate.
In addition to the recognition and incorporation of more exchange channels in the overall climate system, we have come a long way in our ability to measure climate. That is, the resolution of the Earth's climate system has increased greatly over the last 20 years.
However, as impressive as this is, is still not very good resolution. at a grid size of 110 x 110 km, climate models can still not represent clouds and cloud patterns at all. the potential role of cloud feedbacks (to be discussed) is very large. for climate models to properly deal with clouds, the grid resolution will have to shrink to 10 x 10 km. the surface area of the earth is roughly 500 million square km and hence, 10x10 km resolution means 5 million individual grids – we do not yet have the computational power to handle such data volume.>
Because of these kinds of limitations, we are still quite unsure of the exact atmospheric temperature sensitivity to increasing carbon dioxide content. These sensitivities are measured in units of CO2 doubling – that is, how much will the average temperature increase if we double the CO2 content of our atmosphere. The range of plausible values is shown below:
Currently, we are at 420 ppm of equivalent CO2 when including methane (to be discussed in detail later). Even with aggressive attempts at CO2 stabilization (again to be discussed later), we are likely to get to 550 ppm by the year 2050. Referring to the graph above, that would mean a temperature increase in the range of 1.5 – 4 degrees C by the year 2050 – the upper limit of this range is somewhat catastrophic. The range of temperature increases as a function of CO2 stabilization level is shown here:
The basic set of predictions from the 4th IPCC Panel is essentially:
