Comment on the Article “Beyond 2°C: Redefining Dangerous Climate Change For Physical Systems” By Lenton 2011

I have been alerted from Judy Curry insightful ‘s post on Climate Etc titled

Redefining dangerous climate change

to a new paper

Lenton, Timothy M., 2011: Beyond 2°C: redefining dangerous climate change for physical systems. WIREs Clim Change 2011 2 451–461 DOI: 10.1002/wcc.107.

The abstract of this paper reads [highlight added]

“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.WIREs Clim Change 2011 2 451–461 DOI: 10.1002/wcc.107″

The recommendation to broaden out the assessment to other human climate forcings in addition to CO2 and a few other greenhouse gases has been emphasized in a variety of publications which, unfortunately, have been mostly ignored in the IPCC assessments as well as by most policymakers.  There is also the need for a more accurate understanding of natural climate forcings and feedback as discussed, for example,  by Judy Curry and Peter Webster  on Climate Etc, Anthony Watts and others on Watts Up With That, and Joe D’Aleo on ICECAP).

The need for a broader view has been reported, for example, in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.

McAlpine, C.A., W.F. Laurance, J.G. Ryan, L. Seabrook, J.I. Syktus, A.E. Etter, P.M. Fearnside, P. Dargusch, and R.A. Pielke Sr. 2010: More than CO2: A broader picture for managing climate change and variability to avoid ecosystem collapse. Current Opinion in Environmental Sustainability, 2:334-336, DOI10.1016/j.cosust.2010.10.001.

Kabat, P., Claussen, M., Dirmeyer, P.A., J.H.C. Gash, L. Bravo de Guenni, M. Meybeck, R.A. Pielke Sr., C.J. Vorosmarty, R.W.A. Hutjes, and S. Lutkemeier, Editors, 2004: Vegetation, water, humans and the climate: A new perspective on an interactive system. Springer, Berlin, Global Change – The IGBP Series, 566 pp.

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp.

In the 2005 National Research Council report, we wrote

“Regional variations in radiative forcing may have important regional and global climatic implications that are not resolved by the concept of global mean radiative forcing. Tropospheric aerosols and landscape changes have particularly heterogeneous forcings.’


“Several types of forcings—most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry—impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere and by modifying the amount and types of vegetation. No metrics for quantifying such nonradiative forcings have been accepted. Nonradiative forcings have eventual radiative impacts, so one option would be to quantify these radiative impacts. However, this approach may not convey appropriately the impacts of nonradiative forcings on societally relevant climate variables such as precipitation or ecosystem function. Any new metrics must also be able to characterize the regional structure in nonradiative forcing and climate response.”

As we wrote in our 2009 EOS paper

“The evidence predominantly suggests that humans are significantly altering the global environment, and thus climate, in a variety of diverse ways beyond the effects of human emissions of greenhouse gases, including CO2. Unfortunately, the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment did not sufficiently acknowledge the importance of these other human climate forcings in altering regional and global climate and their effects on predictability at the regional scale. It also placed too much emphasis on average global forcing from a limited set of human climate forcings. Further, it devised a mitigation strategy based on global model predictions. For example, although aerosols were considered as a global average forcing, their local effects were neglected (e.g., biomass burning, dust from land use/land cover management and change, soot from inefficient combustion). Because global climate models do not accurately simulate (or even include) several of these other first-order human climate forcings, policy makers must be made aware of the inability of the current generation
of models to accurately forecast regional climate risks to resources on multidecadal time scales.”

“We recommend that the next assessment phase of the IPCC (and other such assessments) broaden its perspective to include all of the human climate forcings. It should also adopt a complementary and precautionary resource- based assessment of the vulnerability of critical resources (those affecting water, food, energy, and human and ecosystem health) to environmental variability and change of all types. This should include, but not be limited to, the effects due to all of the natural and human caused climate variations and changes.”

The new Lenton paper is a start in the right direction.

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