(e.g., mid-Miocene, Late Jurassic, Early Cretaceous) will require careful integration of paleoatmospheric CO2 and paleotemperature time series with improved temporal resolution, precision, and accuracy, as well as data-model comparisons to critically evaluate the veracity of these apparent mismatches. With these improved data, a hierarchy of models can be used to test various forcing mechanisms (e.g., non-CO2 greenhouse gases, solar, aerosols) to determine how well mechanisms other than CO2 can explain anomalously warm and cold periods and to critically evaluate the climate processes and feedbacks that led to particular climate responses characteristic of greenhouse gas-forced climate changes in the past.

Climate Dynamics of Hot Tropics and Warm Poles

Paleoclimate observations provide a conundrum that must be resolved to understand the climate system—the evidence that past temperatures in the tropics and polar regions were periodically much hotter than today. How can the Earth maintain tropical temperatures approaching 40°C, or how can polar temperatures remain above freezing year-round? Yet there is very strong evidence for both conditions during past warm periods. The deep-time paleoclimate evidence suggests that the mechanisms and feedbacks in the modern icehouse climate system that have controlled tropical temperatures and a high pole-to-equator thermal gradient may not apply straightforwardly in warmer worlds. Moreover, the fundamental mismatch between climate model outputs, modern observations, and paleoclimate proxy records discussed in Chapter 2 highlight the degree to which science’s current understanding of how tropical and higher-latitude temperatures respond to increased CO2 forcing remains limited. An improved understanding of these processes, which may drive significant changes in surface temperatures in a future warmer world, is imperative given the potential dire effects of higher temperatures on tropical ecosystems and the domino effect of polar warming on ice sheet stability, the stability of permafrost (which carries a large load of greenhouse gases), and regional climates through atmospheric teleconnections with the tropics and/or polar regions.

Accomplishing this goal requires that the range of deep-time observational data be expanded to include latitudinal transects that span the tropics through mid- to high-latitude regions for targeted intervals of Earth history. Improved constraints on the meridional thermal structure of warm worlds will require increased chronological constraints and more spatially resolved proxy time series than currently exist. New theoretical and modeling approaches are also required to develop a comprehensive understanding of the limits of tropical and polar climate stability, and an understanding of how a weaker thermal gradient is established and main-



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