January 29, 2016

What if climate skeptics are right about climate sensitivity?

Climate skeptics frequently predict that the real climate will warm less than climate models suggest it will over the next century. This, they argue, reduces the need to respond to climate change with policy interventions. Does it really? Let’s suppose climate skeptics are right about climate sensitivity. That would reduce the urgency of responding to climate change and reducing CO2 emissions, but it would not diminish the case for policy intervention.

If you accept that carbon dioxide is a greenhouse gas and that human fossil fuel use is now the dominant contributor to atmospheric CO2 changes, then knowing how much global temperatures respond to increased greenhouse gases in the atmosphere is important for understanding the future climate.

The degree of warming that is caused by increasing greenhouse gases is known as climate sensitivity. Scientists often talk about it in terms of the equilibrium climate sensitivity (ECS), which is the long-term temperature increase that we expect from a permanent doubling of atmospheric CO2.

Climate skeptics frequently argue that the values of climate sensitivity that are reported by the IPCC and displayed by climate models are too high. Using multiple lines of evidence, the IPCC reports that ECS is likely (2/3 chance) in the range of 1.5-4.5 °C. Many skeptics—in particular, a group called “lukewarmers,” for instance Matt Ridleyclaim that it is on the lower end of that range and therefore climate change is less of a worry.

We already looked at how climate skeptics rely on a selective reading of the literature to highlight low estimates of climate sensitivity and use the divergence between climate models and measured temperatures to make conjectural statements about climate models being too sensitive to CO2, without considering other factors that could account for such divergence. Neither has satisfactorily shown that low climate sensitivity is certain. On the other hand—to be fair—evidence for higher climate sensitivity does not disprove estimates of lower sensitivity.

So what if skeptics are right and climate sensitivity is on the lower end of the IPCC range? Let’s examine the implications of a low climate sensitivity for two policy-relevant quantities: temperatures increase in the 21st century and the expected economic costs of climate change (the social cost of carbon).  As a representative low estimate, we’ll adopt an ECS estimate of 1.64 °C with a likely range of 1.25-2.45 °C, an estimate which comes from this 2014 paper by Lewis and Curry. We’ll compare that to an ECS estimate of 3 °C, which is at the center of the IPCC likely range (1.5-4.5 °C).

How Hot Will it Get?

Temperature projections for the next century hinge both on natural and human factors. Climate sensitivity is a natural property of the climate system, but fossil fuel use is a matter of human choice. To estimate future temperature, we need to estimate what future CO2 emissions will be – even in a low sensitivity world.

To cover the range of possible energy futures, the IPCC’s 5th assessment considered scenarios for future economic development and energy policy, known as Representative Concentration Pathways. Figure 1 examines 21st century temperature for two of those scenarios, RCP6 and RCP8.5. Present policy probably sits somewhere between these two scenarios. The trailing number indicates the degree of human influence on the climate. In RCP8.5, atmospheric CO2 grows to nearly 1000 parts per million (ppm), from about 400 ppm today. In RCP6, levels peak closer to 600 ppm in the 21st century. The other two scenarios reflect worlds with steep declines in CO2 emissions over this century. Those scenarios are excluded from this analysis, because we are trying to understand if a low-sensitivity world means CO2 emissions can be high.


Figure 1: If climate skeptics are right about climate sensitivity (green), then global average temperature increases will be more moderate this century, shown here for RCP6 (left) and RCP8.5 (right). Orange lines show temperature increases for a climate sensitivity of 3 °C (1.5-4.5 °C orange envelope) and green lines show temperature for 1.64 °C (1.24-2.45 °C green envelope). Calculations performed with the publically-available climate model, MAGICC (run online here).

As one would expect, a lower climate sensitivity significantly reduces temperatures in the later part of the century. The temperature increase at the end of the century in the low sensitivity case is about half that of the IPCC-range estimate.

Does That Mean Additional Climate Policy is Unnecessary?

The international community has agreed on a goal of preventing warming in excess of 2 °C over the preindustrial era (with an ambitious goal of 1.5 °C). The 2 °C target is a matter of substantial debate (too weak, too strong, too vague…), but it is a good yardstick for understanding the scale of climate change, and its impacts, that we are setting ourselves up for this century.

If climate skeptics are wrong and sensitivity is closer to the middle range, both scenarios above will see warming greater than 2 °C. Even with a low climate sensitivity, high emissions scenarios will lead to warming exceeding the nominal 2 °C target. If skeptics are right and we follow the high emissions scenario, we’ll break that limit 3 decades later than we would otherwise suspect.

If climate skeptics are right and we follow the moderate emissions scenario, then we could well stay below 2 °C. For context, RCP6 is a policy-intervention scenario, where global CO2 emissions peak around 2060 and decline thereafter, because of a steeply increasing carbon tax instigated at mid-century. But it is an illustrative scenario: its price and policy pathway does not reflect the current policy situation. Carbon dioxide emissions are presently larger than the RCP6 case and are tracking the RCP8.5 case.

So how close will current policy measures take us toward an RCP6-like world?  The 2015 Energy and Climate Outlook from the Massachusetts Institute of Technology (MIT) might give a clue. It provides an outlook for global development and carbon emissions for the 21st century under current policy, including the pledges that were made for the Paris Climate Conference in December.

The MIT group projects that current policies will lead to CO2 levels greater than those in RCP6 (but not by a huge amount). Their estimates show that current policies, at a low climate sensitivity of 2 °C, will increase global temperatures by about 1.9 °C at mid-century and 3.1 °C by the end of the century. The report concludes that “even with low climate sensitivity, on this path, the 2°C target will be passed shortly after mid-century.”

The MIT group uses a higher estimate of climate sensitivity (by half a degree) than some skeptics would like, but this calculation does a better job than the illustration I gave above (for a variety of technical reasons).

The general picture is that current policy would have to work as intended (not a guarantee in the United States or internationally) and climate sensitivity would have to be very low to prevent exceeding the international target of 2 °C (let alone, 1.5 °C, which will be a policy and technological challenge even in a low-sensitivity world).  

How Much Damage Do Carbon Emissions Cause?

Another way to look at the implications of a low climate sensitivity is to consider the Social Cost of Carbon (SCC). This is the financial damage expected from the emission of a marginal ton of carbon dioxide. Models that link climate change to economic and social factors (called Integrated Assessment Models or IAMs) estimate the SCC. The value depends strongly on social judgments, economic modeling, and on the physics of the climate. Given uncertainty and subjectivity, credible estimates can run from a few dollars per ton of CO2 to hundreds of dollars per ton.

Accordingly, let’s look at the relative changes in SCC between low and high climate sensitivity to see what it would mean if we really live in a low-sensitivity world.

Climate sensitivity is a physical driver of the SCC. When climate sensitivity is high, the SCC goes up because the climate responds strongly to CO2 emissions and causes more damages. Likewise, when climate sensitivity is low, emissions drive less temperature change and cause less damage, leading to lower estimates of the SCC.

In a 2013 blog post, Chris Hope of Cambridge University and a leading climate-economics modeler showed how lower estimates of climate sensitivity affected the SCC. Using his PAGE09 model, he calculated an SCC of about $100 per ton of CO2 for generally accepted values of climate sensitivity (Hope does the analysis for transient climate response, a measure of climate sensitivity that is technically distinct from the ECS, but the effects on price should be similar).

Using the results of Otto et al 2013—a study that found low-end climate sensitivity and is frequently cited by climate skeptics—Hope calculates a reduction in the SCC of about 45% for a low climate sensitivity world. Hope concludes that, if we’re in a low-sensitivity world, that would imply an SCC of about $65 dollars per ton of CO2. Says Hope:

As the UK’s carbon floor price is currently £16 per tonne, and that is only paid by the most energy intensive firms, we can indeed say that the new estimates of the TCR do show that our climate policy is misguided, but not in the direction that Matt Ridley believes; the new TCR estimates support a much tougher policy than the one we currently have in place.

I repeated Hope’s calculations, but for the values of ECS we considered above, using William Nordhaus’s DICE model. Nordhaus’s preferences for social values and his model of economic growth and climate damages suggests an SCC of about $18 per ton of CO2 (for an ECS of 2.9 °C). Substituting in an ECS of 1.65 °C, one calculates an SCC of about $8 per ton of CO2, a reduction of about 55%.

PAGE09 and DICE2013 have different models of the climate-economics interface and different assumptions about social values, but they agree on what low climate sensitivity does in relative terms to the social cost of carbon. In sum, it reduces the SCC by about half, but does not push it to zero when everything else is held constant.

Where does this leave us?

One hopes that the climate skeptics are right and climate sensitivity is low. That would mean that emissions reductions could progress more slowly and still keep climate change within reasonable limits. It would also mean the damages resulting from carbon dioxide emissions would be less for every ton of CO2 emitted (~20 tons of CO2 annually per capita in the US).

But low climate sensitivity, even those as low as climate skeptics champion, does not relieve the need for policy intervention. Carbon dioxide emissions would still disrupt the climate and lead to damages. Likewise, the current policy outlook indicates that warming would still exceed 2 °C in the second part of this century—a result that will be more likely if climate is slightly more sensitive than the lowest credible estimates or if politicians’ pledges to reduce emissions do not bear out.

Standing up a carbon tax in the United States will affect the economy. Balancing that with the reductions in future climate damage is a part of the deal. We want to be careful about overpricing carbon and underpricing carbon. If climate skeptics are right, overpricing carbon would lead to too ambitious a policy. If skeptics are wrong, then underpricing would lead to too lax a policy. As we make the argument for carbon pricing, as the most effective way to reduce emissions, it is important to consider the full range of scenarios.