A NEW GLOBAL TEMPERATURE CURVE FOR THE PHANEROZOIC

A new global temperature curve illustrates the change in global average temperature during the last 540 million years. This diagram updates and replaces the temperature curve of Scotese et al. (1999) based on Frakes (1992). The new curve combines geological data and oxygen isotope data. Lithologic indicators of climate such as evaporites, tillites, and coals can be used to map ancient climate zones (Tropical Everwet, Arid, Warm Temperate, Boreal Tropical, Cool Temperate, and Polar; Boucot et al., 2013). During the Phanerozoic the width of these climatic zones changed as the Earth’s climate moved from hothouse to icehouse conditions. Oxygen isotope measurements can accurately estimate the temperature of subtropical surface waters, which varied from 25˚C (77˚F) to 32˚C (90˚F) during icehouse/hothouse times. By combining global temperature estimates from geological data with estimates of tropical temperatures obtained from oxygen isotope studies, it is possible to produce an estimate of the global average temperature for any time in the past. In summary, the modern global average temperature is 14.5˚C (58˚F). The average global temperature for the last 540 million years is ~20˚C (68˚F), but the temperature has fluctuated between 25˚C (77˚F)(hothouse) and 10˚C (50˚F)(icehouse). During the Permo-Triassic Extinction, the global temperature spiked above 28˚C (82˚F). Most of the time, the global temperature gently rises and falls in response to gradual changes in orbital and solar parameters, ocean currents, sea level, atmospheric chemistry (greenhouse gases), and other factors. These changes occur over millions of years. Rarely, there is a drastic change in one of these factors resulting in either rapid global warming (Kidder–Worsley events) or rapid global cooling (Stoll-Schrag events). These abrupt climate excursions take place over thousands of years, rather than millions of years. We are currently living through the beginning of a Kidder-Worlsey event that may ultimately raise the global average temperature to ~20˚C (68˚F). In 5000-10,000 years, the global temperature will return to somewhat cooler (~18˚C , 64˚F) equilibrium state. An animation of changing climatic zones and global average temperature (0-540 million years) can be viewed at: https://www.youtube.com/watch?v=Oo1vbGCUR8s .