“A collection of research results have been published in the peer-reviewed scientific literature in recent months that buoys my hopes for a low-end climate sensitivity.”
One of the key pieces to the anthropogenic climate/environment change puzzle is the magnitude of the earth’s climate sensitivity—generally defined as the global average temperature change resulting from a doubling of the atmospheric concentration of carbon dioxide (CO2).
One of the reasons that the “climate change” issue is so contentious is that our understanding of climate sensitivity is still rather incomplete. But new research efforts are beginning to provide evidence suggesting that the current estimates of the climate sensitivity should be better constrained and adjusted downwards. Such results help bolster the case being made by “lukewarmers”—that climate change from anthropogenic fossil-fuel use will be moderate rather than extreme, and that an adaptive response may be more effective than attempts at mitigation.
In its Fourth Assessment Report (AR4), released in 2007, the Intergovernmental Panel on Climate Change (IPCC) provided this general guidance on the climate sensitivity:
[The equilibrium climate sensitivity] is likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.
In IPCC parlance, “likely” means an expertly assessed likelihood of an outcome or result with greater than a 66% chance of occurrence. “Very unlikely” means less than a 10% change of occurrence.