by Judith Curry
There is some “buzz” about a new paper and essay by Timothy Lenton on dangerous climate change.
Beyond 2C: redefining dangerous climate change for physical systems
Wiley Interdisciplinary Reviews Climate Change
Abstract: Most efforts to define a level of dangerous anthropogenic interference (DAI) with the climate system are framed in terms of global annual mean surface temperature change, with 2°C above preindustrial being the most widely accepted climate policy ‘target’. Yet, no actual large-scale threshold (or ‘tipping point’) in the climate system (of which there are probably several) has been clearly linked to 2°C global warming. Of those that can be indirectly linked to global temperature change, the dangerous levels are necessarily imprecise and vary, with estimates ranging from ∼1°C above preindustrial upwards. Some potential thresholds cannot be meaningfully linked to global temperature change, others are sensitive to rates of climate change, and some are most sensitive to spatial gradients of climate change. In some cases, the heterogeneous distributions of reflective (sulfate) aerosols, absorbing (black carbon) aerosols, and land use could be more dangerous than changes in globally well-mixed greenhouse gases. Hence, the framing of Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC), in terms of stabilization of greenhouse gas concentrations (within a time frame), is too narrow to prevent some types of DAI. To address this, a reframed policy objective is proposed; to limit the overall magnitude, rate of change, and spatial gradients of anthropogenic radiative forcing, and resultant climate change, through restriction of emissions of anthropogenic aerosols, patterns of land use, and concentrations of short-lived, as well as long-lived, greenhouse gases.
Targets to limit the global temperature rise won’t prevent climate disruption. Tim Lenton says that policy-makers should focus on regional impacts.
Now that is a statement I can agree with. More that I agree with:
Global average warming is not the only kind of climate change that is dangerous, and long-lived greenhouse gases are not the only cause of dangerous climate change. Target setters need to take into account all the factors that threaten to tip elements of Earth’s climate system into a different state, causing events such as irreversible loss of major ice sheets, reorganizations of oceanic or atmospheric circulation patterns and abrupt shifts in critical ecosystems.
Such ‘large-scale discontinuities’ are arguably the biggest cause for climate concern. And studies show that some could occur before global warming reaches 2 °C, whereas others cannot be meaningfully linked to global temperature.
When Lenton gets to “what should we do about it,” he misses the mark, IMO:
I suggest that the UNFCCC be extended. The climate problem, and the political targets presented as a solution, should be aimed at restricting anthropogenic radiative forcing to limit the rate and gradients of climate change, before limiting its eventual magnitude.
The beauty of this approach is that it opens separate policy avenues for different radiative-forcing agents, and regional treaties to control those with regional effects. For example, hydrofluorocarbons emissions could be tackled under a modification of the 1987 Montreal Protocol, which aimed to halt ozone depletion. And emissions of black-carbon aerosols and ozone-producing gases could be regulated under national policies to limit air pollution. This would both break the political impasse on CO2 and help to protect vulnerable elements of the Earth system.
While his proposals acknowledges that other elements of radiative forcing are relevant to climate change (mainly anthropogenic aerosols), he fails to include natural climate variability (e.g. forcing by the sun and volcanoes) and of course unforced natural internal variability. Not to mention land use changes, which arguably have a dominant influence on regional climate change.
So is anyone listening to Timothy Lenton?
I have to admit that I had never heard of Timothy Lenton before this article (or if i did, the name didn’t stick in my brain). However, on Lenton’s web page, I spotted this sentence that piqued my curiosity: “I was flattered to be listed among the ten most respected climate scientists by the Financial Times.” So I went to the Financial Times, this links to an article entitled “Top climate scientists share their outlook” which is basically a top ten list. The date on the article is interesting: November 20, 2009 (the climate world as it was right before the climategate impact was felt.) The top ten on this list are . . . (drum roll):
- Stefan Rahmstorf
- John Mitchell
- Rachendra Pachauri
- Myles Allen
- Tim Lenton (described as James Lovelock’s annointed successor)
- Kevin Trenberth
- Chris Rapley
- Susan Solomon
- Carl Wunsch
- Isaac Held
- Richard Lindzen
My one word reaction: Huh?
JC’s perspective on the dangerous climate change issue
On a previous thread “What constitutes dangerous climate change?”, my arguments were summarized as:
In my opinion, the primary reason that the UNFCCC has been unable to define “dangerous” anthropogenic climate change is because they have framed the problem and its solution to be irreducibly global. If the problem is viewed as an aggregate of regional problems in the context of a bottom-up incremental policy approach such as that promoted by Ron Brunner, then presumably a more meaningful understanding of dangerous climate change could emerge, which would be a source of political will for actually addressing the problems.
More importantly, extreme weather events and natural climate variability have adverse impacts, which in some instances (time-space) may counter the impacts of AGW and in others may amplify the AGW impacts. Addressing the regional impacts of natural variability in combination with possible AGW impacts would increase overall resilience to extreme weather events and climate change/variability.
IMO, the IPCC WG2 should be addressing the social vulnerability aspect in a major way, other than relegating this to a single chapter. Unfortunately, the IPCC WG2 seems more intent on attributing adverse impacts of extreme weather events and climate variability to AGW (which the IPCC was roundly criticized for in terms of its statements of confidence by the IAC report).
In my congressional testimony, I discussed climate change winners and losers:
A view of the climate change problem as irreducibly global fails to recognize that some regions may actually benefit from a warmer and/or wetter climate. Areas of the world that currently cannot adequately support populations and agricultural efforts may become more desirable in future climate regimes.
A serious assessment is needed of vulnerabilities, region by region, in the context of possible climate change scenarios, demographics, societal vulnerabilities, possible adaptation, and current adaptation deficits. . . This is the kind of information that is needed to assess winners and losers and how dangerous climate change might be relative to adaptive capacities.
JC summary: Given that the words “dangerous climate change” feature prominently in the UNFCCC treaty, we should figure out what these words actually mean or at least try to come up with an operational definition in the context of the treaty. The continued failure to do so (for lack of even trying) is just astonishing. Most of the effort undertaken seems to be attempting to sell spurious links between death and destruction from tornadoes, tsunamies, etc to climate change. To his credit, Lenton puts the “dangerous” issue on the table for discussion, which is a step in the right direction.