There has been plenty of recent news noting that 2010 saw an incredible jump in global carbon emissions, and so I thought this would be a good opportunity to write a post about the implications of future fossil fuel burning in a longer-term, geologic context. There has been a “paradigm shift” in recent years amongst the climate science community in thinking about how global warming will evolve in the future (depending on the socially driven carbon emissions pathway that we follow). These advancements have come primarily through increased knowledge of the global carbon cycle and the “fate” of CO2 once it enters the atmosphere. I wanted to share some of these results.
The Last Few Years
Because of the global financial and economic crisis that started in 2008, there was roughly a 1.3% decrease in global CO2 emissions from fossil fuel burning in 2009. However, 2010 witnessed a record year for global CO2 emissions from fossil-fuel combustion and cement manufacture. Globally, over 9.1 billion tons (or 9.1 gigatons) of oxidized carbon was emitted from these sources, an increase of roughly 6% from the previous year (that is also the same as 33.5 Gt of CO2, after accounting for the mass of the oxygen atoms). The previous record year was 2008, when 8.75 Gt C was emitted.
A substantial part of the global increase from 2009 to 2010 is due to increased emissions from the world’s largest fossil-fuel emitter, China, where emissions rose ~10%. Other developing nations such as India are also growing rapidly, and total emissions from developing worlds are actually expected to exceed those in the developed world within this decade.
Our Future: Emissions, Concentrations, and Global Temperature
Once CO2 is put in the atmosphere, it can begin to warm the planet by trapping some of Earth’s energy and inhibiting the rate at which the planet loses heat to space. Because of this, the traditional approach to estimating the severity of global warming was to determine how much the planet warmed for a given change in atmospheric CO2 concentration. For example, one could ask how much the planet would warm every CO2 doubling? Unlike emissions, atmospheric concentration isn’t usually measured in gigatons, but rather is expressed as a ratio like “400 CO2 molecules for every one million air molecules,” sometimes seen written as “400 ppm,” which is actually close to the modern concentration. It turns out that the global temperature is expected to rise by about 5°F for every doubling (e.g., going from 200 to 400 ppm, or from 400 to 800 ppm). This metric is often referred to as the climate sensitivity, and determining its magnitude has been one of the grand questions of climate science over the last several decades. It might be 4°F, or it might be 8°F; better constraining climate sensitivity remains a challenge.
Over the last few years, some have suggested that we could think of climate sensitivity in terms of emissions of carbon, rather than the resulting atmospheric concentration. Physically, it’s the concentration of CO2 in the atmosphere that matters for global temperature, but cumulative (total) emissions provide a tractable, concise, and well-constrained metric that may be of use to policy-makers interested in avoiding some threshold peak of global warming. To make this a useful metric however, we need to understand what happens to CO2 once it enters the atmosphere. After all, not all of the carbon stays in the atmosphere—trees and plants take some up, and the ocean also takes up some of the excess carbon. Indeed, one of the other problems with increasing atmospheric CO2 is a phenomena called ocean acidification, a lowering of the ocean pH as a result of the oceans trying to suck up some of the excess atmospheric CO2.
There are a number of recent improvements in our understanding of these processes, some of which may be surprising, which are summarized in bullet points below:
- Once the atmospheric CO2 concentration is perturbed (say, increased from 300 to 500 ppm), it takes centuries for most of the excess carbon to leave the atmosphere. Some of the CO2 is taken up by plants and by the ocean over many decades, while at least 25% of the extra “pulse in CO2” will remain in the atmosphere for tens of thousands of years. This makes global warming a geologic-scale problem, not just one for current generations.
- The amount of global warming that you get in the future increases roughly linearly with peak cumulative carbon emissions. The warming is approximately 3.2°F (1.8°C) for every billion tons of carbon emitted (or 3.7 trillion tons of CO2), although this rather uncertain, and has some sizable error bars, as shown below. For example, to get a temperature increase of 2°C (or 3.6°F) might require 0.8 trillion tons of carbon, or it might require 1.5 trillion tons. This uncertainty is due to a lack of full understanding of how fast the ocean or biosphere sinks take up excess atmospheric carbon, as well as how sensitive the climate system is to change. That depends on a number of feedback processes (see here), such as how clouds might change in a warmer climate, that could potentially amplify or dampen the warming.
- Stabilizing climate at a given target CO2 concentration (e.g., 450 ppm, 550 ppm) would require up to 80% reduction in global carbon emissions! Even more than this is required to stabilize global temperature in the long-term.
- Once emissions of human CO2 cease, global temperature still remains elevated above the pre-industrial value for something like ~1,000 years, and thus climate change is irreversible on human timescales (unless we could find a way to artificially remove CO2 from the atmosphere, which is not currently feasible). This is shown below in Figure 2. Figure 2a shows CO2 emissions growing at a rate of 2%/year to peak CO2 values of 450, 550, 650, 750, 850, and 1200 ppm, followed by zero emissions. The CO2 concentration decays gradually, but the global temperature (Figure 2b) remains steady for at least a millennium. The reason for this involves the enormous heat capacity of the deep ocean.
Figure 2- a) Climate system responses are shown for a ramp of CO2 emissions at a rate of 2%/year to peak CO2 values of 450, 550, 650, 750, 850, and 1200 ppmv, followed by zero emissions, and the falloff of CO2 concentrations following zero emissions after the peak. b) Corresponding surface warming for each of the above scenarios (units are in Kelvin, multiply temperature changes by 1.8 to have the equivalent value in Fahrenheit). From Solomon et al., 2009, PNAS
Because of the very long atmospheric lifetime of the excess CO2 (decades to centuries), global warming becomes a long-term problem. A geologist looking back a million years from now might see the climatic impacts of humans on the same scale as the coming and going of an ice age. The numbers being talked about (for example, a 5°F increase in global temperature) have not actually occurred on a global scale for at least a few million years. In fact, the last time CO2 levels were comparable to modern concentrations happened around 3 million years ago; global temperature was a few degrees warmer, and sea levels were ~25 meters higher than modern, indicating substantially reduced Greenland and West Antarctic ice sheet sizes.
In a policy setting, it is sometimes argued that slow processes such as climate change will pose a small risk, on the basis of the assumption that a choice can always be made to quickly reduce emissions. However, as shown above, this is not true. If atmospheric concentrations were stabilized at present values (requiring ~80% reduction in emissions), there would still be some inevitable future “warming in the pipeline” as the ocean warms in an attempt to “catch up” with the modern atmospheric composition. This is a result of the thermal inertia of the oceans, but there is also social and policy inertia to consider. As highlighted in this post, even if emissions were to cease altogether so that CO2 concentrations actually declined a bit, you could eliminate that warming in the pipeline but the planet would still be warmer than pre-industrial conditions for several generations. During those many centuries of elevated temperatures, a number of “slow responses” (like ice sheet disintegration) that only come to equilibrium on very long timescales, would have time to adjust to the new climate.
It also follows from this that any attempt for a “fast action” solution to cool global temperature would need to be sustained for centuries (some ideas, such as pumping reflective particles in the air to block sunlight, no longer work when the project is terminated, even though CO2 remains in the atmosphere for centuries). These are all important points to think about when deciding what climate we want for later generations, since our past decisions, and certainly our present ones, will stick around for the future.