David Lea, professor in UC Santa Barbara's Department of Earth Science delivered a special seminar titled Can Global Warming Be Held to 2 Degrees Celsius above Pre-Industrial?

This talk examines two related questions:
How feasible is to limit global warming to 1.5 or 2 deg C?
To what extent is it possible to establish a particular warming level as dangerous?

The first question can be readily addressed, because the main factors and uncertainties that determine future warming are either well characterized or can be specified: i.e., future emission pathways of GHGs and aerosols, climate sensitivity (i.e., strength of climate feedbacks) and the current cooling effect of aerosols, and earth system responses such as carbon cycle feedbacks. A critical economic constraint on the feasibility of limiting warming is the maximum possible rate of absolute emission reduction, which is limited by how rapidly energy infrastructure can be turned over.

The second question regarding what is dangerous is more challenging because it involves vast uncertainties in climate projections, the behavior of so-called “tipping elements” in the climate system, ability to adapt to climate change, and subjective judgments about the importance of climate impacts and especially climate extremes. One compelling argument for limiting warming to a specific level centers on the identification of threshold temperatures for specific tipping elements in the climate system that, if tipped, would have large societal impacts. But identifying such thresholds is no simple matter.

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