Beyond Mean Climate: Quantifying Climate Variability and Extremes under Varying Boundary Conditions

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

The Earth’s climate can be defined broadly by two concepts, average values and variability around those averages; for example the average temperature of a location over some interval and how much the temperature varies from that average on any given day, or month, or year, or decade. This variability is important to human societies, driving things like extreme heat waves or droughts. Over Earth’s long history, the average climate has changed in profound ways. For example, 20,000 years ago, the Earth on average was about 5 degrees Celsius colder, and large glaciers covered parts of continents that are ice free today. We know that the average climate has changed because we have evidence from natural archives, such as that found in the chemistry of ice that remains frozen at the poles. But how has the variability changed? Does the variability change with the same pattern as the average? How often do extreme events - such as extremely hot or cold temperatures - occur at different times in the past? Records of the year–to-year or decade-to-decade variability, say from 20,000 years ago, are exceptionally rare. We will use climate records from very high-resolution measurements of ancient polar ice in Greenland and Antarctica, obtained through a process known as ‘ice coring’, to study the inherent variability in Earth’s climate and how it changes over long periods of time, extending backwards in time up to 100,000 years before present. These aspects of climate, the variability and extremes, in addition to the average climate, can provide new contexts that help us better understand our changing planet. The relationship between the mean climate and its internal variability is a fundamental aspect of climate dynamics. Understanding the dependence of internal variability on the mean state is key to understanding the detectability of forced changes in climate and, critically, the change in likelihood of extreme events. Information about internal climate variability throughout Earth’s history, as the mean climate has undergone large changes, such as during the Last Glacial Maximum about 20,000 years ago, is exceedingly rare. Most paleoclimate archives lack the detail or continuity to reliably resolve annual, interannual, and decadal timescales for tens of thousands of years. We will use a suite of five extremely high-resolution ice core records of water isotopes, as well as impurities like dust, to statistically catalog variability and extremes in polar climate and its relationship to the background mean state. We will examine these relationships over periods of stable mean climate, long term changes in global mean climate, as well as abrupt climate change, and provide an unprecedented analysis of high-frequency, high-latitude climate variability. The analysis will span the last glacial-interglacial cycle, extending backwards in time up to a 100,000 years into the past. High-frequency climate variability has been impactful to humans and societies throughout history. Annual, interannual, decadal, and centennial scale variability has driven changes in habitability leading to the blossoming and collapse of past civilizations. Extreme climate events in particular affect modern quality of life, national security, food and water availability, and ecosystem services (among other concerns). The proposed research on climate variability and extremes of the past, and the relationship to average climate, can provide new contexts that help us better understand our changing planet. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.