Saving the planet: Emissions scenarios, stabilization issues, and uncertainties
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Saving the planet: Emissions scenarios, stabilization issues, and uncertainties “NCAR Summer Colloquium on Climate and Health” NCAR, Boulder, CO. 19 July, 2006. Tom Wigley, National Center for Atmospheric Research, Boulder, CO. Introduction.

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Saving the planet: Emissions scenarios, stabilization issues, and uncertainties“NCAR Summer Colloquium on Climate and Health”NCAR, Boulder, CO.19 July, 2006

Tom Wigley,

National Center for Atmospheric Research, Boulder, CO.


The climate change problem is essentially an energy problem that requires moving away from the use of fossil fuels as our primary energy source.

This will almost certainly require the development of new “carbon-free” (or “carbon neutral”) energy technologies.

To determine the magnitude of this technological challenge we need to know what will happen in the absence of policies to limit climate change, and what a safe level may be for future climate change.


Climate changes observed over the 20th century

Future climate change: the no-policy case

Future climate change: stabilization policies

Future changes in energy production

Carbon-free energy requirements for stabilization

Technology options

The ‘wedge’ concept



Observed temperature changes

5 of the 6 warmest years have occurred this decade.

1998 was unusually warm due to a large El Niño that occurred in 1997/8.


(in the absence of policies to reduce climate change)

The SRES ‘no-policy’ emissions scenarios

  • The Intergovernmental Panel on Climate Change (IPCC) has sponsored production of a set of 40 ‘no-climate-policy’ emissions scenarios for GHGs, sulfur dioxide, and other gases

  • These scenarios are based on a range of assumptions for future population and economic growth, technological change, etc., grouped into four families or ‘storylines’ (A1, A2, B1, B2)

  • The scenarios are published in a Special Report on Emissions Scenarios – hence the acronym SRES

  • Six of these scenarios have been used for detailed climate calculations (A1B, A1FI, A1T, A2, B1, B2)

“Special Report on Emissions Scenarios”, Eds. N. Nakicenovic & R. Swart, C.U.P. (2000)

SRES scenarios: Family characteristics

SRES population projections

Economic growth: per capita GDP

SRES fossil CO2 emissions

Remember, the ‘B’ scenarios focus on sustainable development.

SRES CO2 concentration projections

NOTE: Increasing CO2 is not only a climate problem.

Increasing CO2 makes the ocean more acidic, eventually making it impossible for carbonate shell-producing animals to produce shells.

Extinction of these animals will upset the ocean food chain and could lead to much larger scale extinctions


2000–2100 radiative forcing breakdown

NOTE: Dominant role of CO2

Global-mean temperature projections

Future warming compared with the past


Article 2 of the UNFCCC

Article 2 provides the basis for climate policy. Its objective is …

“stabilization of greenhouse gas concentrations ….. at a level that would prevent dangerous anthropogenic interference with the climate system ….. within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner”.

CO2 stabilization pathways


  • Stabilization of CO2 concentration requires, eventually, very large reductions in CO2 emissions. The arrow shows the reduction in 2050 if we wish to stabilize at 450ppm

  • Since most CO2 comes from energy usage, stabilizing CO2 requires that we need to obtain a large fraction of future energy from carbon-free sources.

  • A key issue is, what should the stabilization level be in order to avoid “dangerous interference with the climate system”?



NOTE: Even in the absence of climate policies, large increases are projected for carbon-free energy


The answer depends on the assumed no-policy baseline scenario (35 possibilities in the SRES scenario set) – and on the chosen concentration stabilization level (also a wide range of possibilities).

This implies a wide uncertainty range.

Carbon-free energy requirements


The blue lines show how much carbon-free energy is already built into the baseline scenarios

In the B1 case, the vertical arrow shows the additional carbon-free energy required to move from the no-policy B1 scenario to the WRE450 stabilization pathway.

Note that B1 is a very optimistic scenario – other baseline scenarios require much greater amounts of additional carbon-free energy.

Extra carbon-free energy needed in 2050 (TW)

* Current carbon-free energy  2TW

Extra carbon-free energy needed in 2050 (TW)


(1) The baseline scenarios show large increases in carbon-free energy even in the absence of climate policies. This limits the options for additional carbon-free energy.

(2) The large amounts of carbon-free energy required for stabilization levels of 450ppm or less will almost certainly require the development of new technologies.

* Current carbon-free energy 2TW


CO2 emissions reduction opportunities

* Includes direct capture from the atmosphere


Pacala & Socolow wedges

A ‘wedge’ is a single existing technology that can be scaled up to reduce CO2 emissions by 1GyC/yr in 2055; or reduce cumulative emissions over 2005–2055 by 25GtC.

Pacala and Socolow claim that a 500 ppm stabilization pathway can be followed, at least to 2055, using existing technology. This is incorrect.

S. Pacala & R.Socolow: Science 305, 968–972 (2004)

Baseline wedges

The flaw in Pacala and Socolow is that they fail to account for wedges already built into the baseline scenario. SRES baselines all contain a large amount of carbon-free energy growth (red arrow) that arises spontaneously, in the absence of climate policy.

Wedges required for stabilization(through to 2055)

Wedges already built into no-policy baseline: neglected by Pacala and Socolow.

Wedges required for stabilization(through to 2055)


  • Pacala and Socolow identify 15 existing technology wedges, each of which could be scaled up to reduce emissions in 2055 by 1GtC/yr

  • However, the total number of wedges required to follow WRE450 to 2055 is between 21 and 49

  • We therefore need to develop new carbon-free energy technologies – probably requiring a massive investment in research, demonstration and dissemination.

Wedges already built into no-policy baseline


Geoengineering (1)

  • Reducing CO2 emissions (“mitigation” -- i.e., moving from the use of fossil fuels as our primary energy source to the use of carbon-free energy technologies) is the standard “solution” to the climate problem. Geoengineering is an alternative approach.

  • Geoengineering aims to offset CO2-induced climate change by deliberately altering the climate system.

  • The earliest suggestion was to inject aerosol-producing substances into the stratosphere to provide a cooling shield – i.e., to produce a human volcano.

Geoengineering (2)

  • The problem with geoengineering as a single solution is that our use of fossil fuels creates two problems, climate change and increasing CO2.

  • Increasing CO2 makes the oceans more acidic and could lead to the extinction of all carbonate shell producing animals in the ocean.

  • As these animals are at the bottom of the food chain, their extinction could lead to the extinction of all life in the ocean.

  • Geoengineering cannot replace mitigation (i.e., the reduction in fossil fuel use), but it may make mitigation easier.

Effect of multiple volcanic eruptions


As an analogy, we consider a case where we know the effects of a known injection of SO2 into the stratosphere, the eruption of Mt Pinatubo (June, 1991)

Pinatubo released 10TgS of SO2 into the stratosphere. This is 15-20% of the current amount of SO2 that we release each year into the troposphere. The eruption cooled the planet by 0.5 – 0.7oC.

Alternative geoengineering scenarios

Scenarios like these produce immediate cooling, offsetting the short-term effects of increasing CO2.

This means that, with geoengineering, we would not have to reduce CO2 concentrations or emissions so rapidly.

This gives us more time to develop alternative, cost-effective carbon-free energy sources.

-3W/m2 is equivalent to Pinatubo every two years (5TgS/yr)

Concentrations and implied emissions

Overshoot is the case that includes geoengineering.

Note how this gives us much more time (around 20 years) to begin the required rapid reduction in CO2 emissions – i.e., more time to phase out existing energy systems and develop alternative technologies.

Geoengineering effects on climate

The important comparison here is between WRE450 (mitigation alone) and LOW, MID or HIGH GEO (geoengineering combined with slower mitigation, but with the same CO2 stabilization level).


  • Global warming is primarily an energy problem.

  • The problem, however, is two-fold – involving both climate change and the effects of increasing CO2 on ocean acidity.

  • Solving both problems requires satisfying very large future energy demands with, primarily, carbon-free energy sources.

  • Projected carbon-free energy requirements are extremely large. They cannot be satisfied with current technology.

  • Large investments are required to develop these new technologies.

  • Moderate intervention using geoengineering could give us time to develop and implement these new technologies.



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