After running across some resources again recently, I thought it would be a good idea to put some posts together that showed the background of many of the common facts I discuss. In this first post, I wanted to show the relationship between greenhouse gases, radiative forcing and temperatures. In doing, I will use graphics from the IPCC’s 4th Assessment Report Technical Summary.
First, here is a graphic of changes in greenhouse gases from ice core and modern observational data, spanning the time period of 20,000 years ago through current:
The portion of this graph I’d like to focus on is the upper left quadrant displaying the time series of atmospheric carbon dioxide concentration. First, note is the transition from ~180ppm 20,000 years ago to between 260 and 280ppm. This transition helped bring the last interglacial period to an end. Of greater import is the more recent transition from 280ppm to 380ppm (as of ~2005; current concentrations are ~390ppm).
The graph’s vertical axis is of the same scale, which shows the incredible magnitude of the recent increase in true historical context. More recent research suggests that the 20th century change in atmospheric CO2 concentration is faster than anything in the past 120 million years. Ecosystems can respond to slow climate change, as happened during the Cretaceous Hothouse, when volcanic eruptions put enough greenhouse gases in the climate system to increase temperatures by 5C over millions of years. No massive life form die-offs occurred largely because the oceans were able to absorb the extra CO2 over time. In contrast, life suffered during the Paleocene-Eocene Thermal Maximum. Global temperatures again increased by 5C, but they did so over only ~20,000 years. That rate of change made all the difference in how life was unable to respond in time before going extinct.
All of which leads to the question: how fast will temperatures increase during this event? Many times faster than the most extreme climate shift ever known. 40 to 160 times faster, in fact. And that is based on 1 to 4C warming over just the next 100 years. Overall, warming of 2 to 10C over the next 200 to 300 years could occur. As previous climate events have proven in the past, that level of warming over such an incredibly short time period will likely prove disastrous for many species.
Which leads me to the next graph – one that explains the physics more clearly: how do natural and human-caused sources affect radiative forcing? Do they add to solar radiation or do they act in opposition to it?
As the top portion of the graph shows, CO2 is clearly the largest magnitude positive radiative forcing constituent. CO2 contributes over 1.5W/m^2 by itself. If methane and N2O are included, another 1W/m^2 is added to the mix. The current best estimate of cloud albedo effects is negative .75W/m^2. The bottom portion shows the relative probability of radiative forcing of total aerosol effects (blue dashed line) and greenhouse gas forcing (red dashed line). As you can see, the greenhouse forcing has stronger forcing and much higher relative probability. The result when all the effects are combined is shown by the solid red curve. The highest probability indicates between 1.5 and 2W/m^2 of total anthropogenic radiative forcing – on top of the positive natural radiative forcing.
The third and last graphic shows what the radically higher CO2 concentrations shown in the first figure have already done to global surface and tropospheric temperatures by way of the additional forcing shown in the second figure.
The globe’s surface warmed at 0.045C per decade over the past 150 years (ending in ~2005). In the past 100 years, that warming is 0.074C per decade. Most recently, over the past 25 years, the surface has warmed at 0.177C per decade. As the graph clearly shows, the rate of warming is increasing. Moreover, the warming is unequivocal. The chemistry and physics involved in these processes have had measurable effects in the past. They are having the same measurable effects in the present. We know with increasing confidence what the first-order effects of the future will be. This warming won’t stop until the emission of greenhouse gas pollution stops.