One common objection to climate policy that would reduce CO2 emissions is that the contemplated emissions reductions will be too modest to affect global temperature. Moreover, we have little capacity to estimate how emissions reductions may benefit human society. How, then, can we be sure that we will actually be reducing climate change risks with carbon policy?

The answer to that objection is two-fold. First, scientists have established a direct relationship between total CO2 emissions and global warming. Second, scientists have established that the risks associated with climate change increase with temperature. While we don’t know where the specific lines are that would turn the manageable into the catastrophic, and we don’t know the sensitivity of the climate to CO2 emissions with precision, the basic relationship between CO2 emissions and risk point suggest that decreasing CO2 emissions is a viable risk management tool.

Temperature goes up with total CO2 emissions

Climate science tells us that there is an approximately linear relationship between cumulative CO2 emissions and global temperature increases. This behavior is found in simple and IPCC-class climate models. Results from the latter are shown in the chart below, where total CO2 emissions are plotted against temperature increases from IPCC climate models.


Figure 1: The linear relationship between cumulative CO2 emissions and global warming as calculated by IPCC-class climate models. Scenarios with other climate forcing agents are shown by the salmon envelope. The grey envelope shows what happens when only CO2 increases.

The linearity between cumulative emissions and temperature comes from opposing phenomena that largely compensate for each other. As CO2 concentrations in the atmosphere increase, the additional radiative forcing response weakens (forcing scales logarithmically with concentration), which would tend to bend the curves in figure 1 downward. However, the ability of the natural system to absorb anthropogenic CO2 from the atmosphere decreases as CO2 concentrations (or total emissions) increase. So for every additional ton of CO2 emitted, a larger fraction resides in the atmosphere and contributes to increased radiative forcing, which would bend the curves in figure 1 upward. These two effects seem to compensate each other. A linear relationship between total emitted CO2 and temperature increases emerges.

The implication is that if one wants to reduce future temperature increases, then reducing total CO2 emissions is necessary.

Climate change risk goes up with temperature.

That increasing temperature increases climate risks is a matter of firm and general agreement. The following chart shows how the IPCC rates risks across areas of concern and how those risks increase with temperature. In no case does risk go down with increasing temperature.


Figure 2: Heat map of different climate risks relating to global temperature via IPCC.

It is worth taking a look at a few of these areas of concern in the course of considering how temperature increases affect risk.

The IPCC reports that the risks associated with extreme weather events (heat waves, extreme precipitation, and the like) are moderately increased with the approximately 1 °C warming that we have already experienced (the recent report from the National Academies would support that conclusion) and that further warming will increase those risks. Some of that risk can be reduced by smart adaptations and economic development to reduce vulnerability. Even so, the increased probability of extremes with changing average conditions is physically and statistically sound.

Global aggregate impacts refer to reductions in the global economy (or global biodiversity). Such impacts are consolidated into integrated assessment models to calculate the social cost of carbon and perform global cost-benefit analyses for climate policy. There is high agreement that risks to the global economy accelerate with temperature increases, although even the IPCC reports that there is limited evidence for how quickly they might accelerate because we don’t really know how to estimate aggregate damages for warming greater than about 3°C.

Large-scale singular events are abrupt or irreversible changes within the climate system. The risk of rapid disintegration of the polar ice sheets is a prime example. The IPCC says that the risks of such an event increase disproportionately between 1 and 2 °C of additional warming and become high with warming above 3°C.

This new paper from James Hansen and others, though controversial, discusses the risks associated with that particular event. It describes a series of feedback mechanisms that would lead to multi-meter increases in sea level rise, even with only 2 °C of warming. That puts this study’s predictions of possible sea level rise well above the results that come from other groups. The IPCC estimates a likely upper bound for sea level rise, for instance, at about a meter, but under much stronger warming scenarios.

The threshold temperatures after which feedback cycles like the one hypothesized by Hansen et al. activate are not well known or studied. But Hansen et al. have laid out a plausible (good enough to pass a very public peer review), if not likely, scenario that would have major consequences for human society. It a good example of the sort of high-impact, low-probability risks that convince many otherwise sober analysts that climate change demands serious policy attention. The best means of reducing the risk of kicking off ice melt feedbacks is to reduce temperature increases.

So for each of these three areas of concern—extreme weather, globally aggregated damages, and large-scale singular events—the evidence (though it may be limited) points toward risks that increase with rising temperatures. And they often do so non-linearly.

If we want to reduce climate risks, then we should aim to reduce temperature increases. And given the linear relationship between warming and emissions, we know that reducing total CO2 emissions will reduce risks. Since we don’t know where the temperature thresholds might be for various climate responses, all we can say for certain is that reducing emissions reduces risk, and the more emissions reduction we achieve, the more risk we avoid.