The Complex Variability of Climate

A huge challenge the research community faces is dealing with uncertainties in future climate projections. Addressing these uncertainties, particularly in the climate’s response to human activity, is a really important issue. And that’s one of the key things that we address in this paper.

There are two basic methodologies, or areas of science, whose use we discuss. From the math side, dynamical systems theory, and on the physics side, statistical mechanics (specifically fluctuation-dissipation and linear response). There are many papers that individually discuss the use of these methods in studying climate, but not one that looks at them both. This paper, written by Valerio and me, attempts to do that.

Unfortunately, there is a perception—perpetuated by some climate science activists—that, “the science is done when it comes to climate change; now society needs to act.” I agree that we do need to act; there’s plenty of evidence for that. But the science is very far from done. We’re only just getting to the really hard, fundamental questions.

Scientists have a good understanding of how human activities are modifying the mean global temperature. But we are missing a full understanding of how they impact other aspects of climate—for example, the amplitude and frequency of extreme events.

One of the issues is that most people, scientists or otherwise, think of climate as something static that anthropogenic factors are just pushing around. But the natural climate constantly fluctuates. What I hope Valerio and I offer in this paper is a unified framework for modeling climate change and climate variability that accounts for these fluctuations.

Weather can be defined as the condition of the atmosphere on a timescale shorter than a season, or about 100 days. The state of the atmosphere and ocean on a timescale beyond ten years is certainly climate. Somewhere in between, the two overlap. Unfortunately, there’s no easy way to distinguish exactly where weather stops and climate starts.

Think about the seasons, whose onward march is the most dramatic natural climate phenomenon. There can be a bigger difference in the weather between summer and winter in places such as New York, for example, where the temperature drops below freezing in winter and rises above $30{}^{\circ }\text{C}$ in summer, than there is between the climate today and a century ago. But the march of the seasons is changing from one decade to the next. And how you enter a winter, for example with a warmer or a colder ocean, may affect the number or intensity of particularly cold episodes.

Global warming is accompanied by a reduction in the equator-to-pole temperature difference, since the temperature is rising more in the polar regions than in the equatorial ones. This reduction can increase the number of extreme events, such as the recent polar-vortex-induced cold snaps in the Midwest and Eastern US. But this conjecture is still hotly debated.

The climate system is complex, and there are unsolved problems in understanding how it behaves that merit attention. The fact is that human activity is affecting more than the mean temperature; we’re affecting the march of the seasons and the amplitude, duration, and frequency of extreme weather events.

Keeping it to only 77 pages and 52 figures.