Burning issues at climate science – policy interface

Judith Curry

Agreement:• Surface temperatures have increased since 1880• Humans are adding carbon dioxide to the atmosphere• Carbon dioxide and other greenhouse gases have a

warming effect on the planet

Disagreement:• Whether the warming since 1950 has

been dominated by human causes• How much the planet will warm in the 21st

21st century• Whether warming is ‘dangerous’• Whether we can afford to radically

reduce CO2 emissions, and whether reduction will improve the climate

Burning issues at the interface between climate science and policy

Social cost of carbon: economic costs of CO2, and benefitsof CO2 reduction

Evolution of 21st climate: how much warming, and when?

Impact of proposed policies on 21st century climate change.

On trial in MN: Social Cost of Carbon

A 1990s law requires the PUC to establish externality values for CO2 and other power plant pollutants to help guide utility planning decisions.

The PUC adopted the federal Social Cost of Carbon (SCC)

SCC is being challenged by energy companies andindustry groups in a current trial

Equilibrium Climate Sensitivity

Definition: Global surface temperature change following a doubling of atmospheric CO2 concentration

Determined from:• global climate models• historical observations• paleoclimate reconstructions

How much warming will emissions reductions commitments prevent in the 21st century?

U.S.: (calculations by CATO: ECS=3oC)

• Reducing U.S. total emissions by 80% by 2050 will prevent 0.11oC in warming by 2100

Global estimates (in progress)

• 0.2oC is optimistic target

• India’s INDC will double - triple emissions by 2030

AR5 2013

The amount of warming depends on the emission of greenhouse gases

Implications for the future: I. IPCC AR5 view

Growing divergence: models vs observations

• Are climate models too sensitive to greenhouse forcing?

• Is modeled natural climate variability inadequate?

• Are model projections of 21st century warming too high?

50 YEARS, 100 YEARS

CO2 emissions

Scenarios of future climate

Solar effects

Volcaniceruptions

Unknowns

Long rangeocean oscillations

Geologicprocesses

Implications for the future: II. View emphasizing natural variability

• The rate of warming will be slow for at least another decade (into the 2030’s?); ocean circulation patterns

• Climate models are too sensitive to human forcing; 21st century warming will be on the low end of IPCC projections (or even below)

• Solar variations & volcanoes: wild card. Some are predicting major solar cooling in the near term

• Can’t rule out unforeseen surprises

What does climate science say about policy and mitigation solutions?

• There is increasing evidence that the threat from global warming is overstated

• However, if the threat is not overstated, there are major shortfalls in current and proposed solutions.