The concentration of atmospheric CO2 has risen from 280 parts per million (ppm)at the dawn of the industrial revolution in the eighteenth and early nineteenth centuries to more than 410 ppm in 2017, the highest level in the past three million years. How sea surface temperature responds to such high greenhouse gas levels is crucial for evaluating near-future climate predictions under the current warming trend of recent decades. The unresolved arguments regarding linear versus nonlinear climate responses hinder our understanding of climate evolution and our ability to evaluate the fidelity of global energy simulations under the current warming stress.

A recent study conducted by Dr. Li Lo, Prof. Chuan-Chou Shen, and Prof. Kuo-Yen Wei of the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC) at the Department of Geosciences compiled data records that indicate the nonlinear response of Pacific Ocean sea surface temperature to greenhouse gases over the past 360,000 years, including the last four glacial-interglacial cycles. The study was published in the journal Scientific Reports on July 24, 2017.

In the study, the team's investigators compiled a record of tropical sea surface temperatures for the southern margin of the Western Pacific Warm Pool. This record was derived from a depth profile of trace elements in the calcite shells of a shallow-water marine organism called planktonic foraminifera in a sedimentary core MD05-2925 (9"20.60' S, 151"27.54' E). The core was drilled off the eastern coast of Papua, New Guinea.

Combined with five previous studies in the Western and Eastern Pacific, the team's findings showed that the sensitivity of the responses to greenhouse gas concentrations rises dramatically by a factor of two to four at atmospheric CO2 levels of over 220 ppm. The study also indicated that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach this threshold level.

Their findings suggest that tropical Pacific Ocean sea surface temperatures respond not only to atmospheric radiative forcing but also to regional oceanic dynamics, topography, and climate boundary conditions. Despite the complicated combination of factors influencing sea surface temperatures, the team's new records imply that the response of ocean sea surface temperatures to greenhouse gases could be even severer than previously thought. Energy conservation and the reduction of carbon emissions are the most urgent and essential action for the sustainability of our green Earth.