Why does climate vary?

The challenge of communicating climate science to the public, and the dangers associated with a scientifically over-simplified approach to the popular and political debate, is something about which I have commented on a number of occasions. Scientific literacy may not be particularly high in the population at large, but appealing to the lowest common denominator risks irritating everyone.

In a recent issue of Eos, the international trade journal for geoscientists, is an interview with one of two editors of a book from the American Geophysical Union titled “Climate Dynamics: Why does climate vary?”. Distinguishing between natural climate change and anthropogenic effects is a hot topic in the Earth science community, and a bugbear of climate deniers who feed on ignorance.

In clear and comprehensible prose, De-Zheng Sun explains how climate varies as a result of uneven solar heating of an atmosphere weighed down by gravity, and the overlapping spatial and temporal scales involved in the motion of the thin layers of air and water covering our rocky orb. I so wish that I could reproduce the entire article, for it would I am sure be appreciated by non-scientists. Instead, I shall extract a few key passages…

“[T]he greenhouse effect is a contributor to the differential heating in the atmosphere that sets air and water in motion. When we enhance the greenhouse effect in the atmosphere by adding carbon dioxide (CO2), what the dynamics of the atmosphere feels is its heating effect. To the degree that climate variability is linked to the motions in the atmosphere and ocean, and to the degree the latter is due to the differential heating to which the greenhouse effect contributes, changes in variability are expected to be the norm of the effect of anthropogenic forcing.”

Sun is attempting here to divert us away from a simplistic ‘add carbon dioxide and the global average temperature rises in direct proportion’ argument. As a first approximation, the relation holds true, but what it describes is a one-dimensional abstraction, rather than the real, complex system that is Earth’s atmosphere and ocean. What happens in the real world is that increased CO2 due to human activity significantly influences the natural variability of the climate system. It changes the magnitude of what is otherwise entirely natural climate variability.

The problem with traditional climate models has been their one-dimensional nature, which fails to adequately simulate feedback mechanisms that result from multi-dimensional processes. This does not mean that the linear CO2-warming relation is incorrect; with the one-dimensional models we may well be underestimating the global warming effect of anthropogenic carbon dioxide and other greenhouse gas emissions…

“Although three-dimensional models have now been developed, the conceptual picture about the effect of an increase in CO2 is still largely underpinned by the results from those one-dimensional models, as is evident in the notion that a significant linear trend in temperature will be the defining feature of global warming caused by anthropogenic enhancement of the greenhouse effect. The continuing influence from these early models over the way people conceptualize the anthropogenic effect is also because the state-of-the-art three-dimensional models still do not properly simulate natural variability such as MJO [Madden-Julian Oscillation] and ENSO [El Niño/La Niña–Southern Oscillation]. As a result, models are not yet able to capture the anthropogenic effect that takes place in the form of climate variability. In other words, our models may be underestimating the effect from anthropogenic forcing on natural variability. It is time to look seriously at an alternative hypothesis, which is that the defining feature of global warming will be changes in the magnitude of climate variability…”

Sun stresses that modelling natural variability is essential when it comes to assessing the anthropogenic component to global warming. For example, we hear a lot in the mainstream media about ‘tipping points’ in the climate system, but little about the atmospheric and oceanic dynamic regimes that influence such feedback mechanisms and points of no return.

“The prevailing conceptual framework that has been used to quantify climate changes stemming from anthropogenic forcing is that increasing CO2 concentration will create a linear trend in temperature and other state variables that define the mean climate. I would suggest that the focus may need to shift from looking for trends in mean temperature to statistical changes in the magnitude and other attributes of natural variability. The effect of anthropogenic forcing is likely to manifest in climate phenomena at all time scales so long as these climate phenomena derive energy from differential heating. This shift in paradigm may further highlight the need for a better understanding of the mechanisms behind natural variability, in particular, their thermal and nonlinear aspects.”

I would recommend that you read the last quoted passage repeatedly, until you have fully absorbed its implications on a conceptual level. On a practical level, one could argue that it is crisis-inducing change, per se, rather than warming or cooling, that humanity should be most concerned with. That the global average temperature is increasing is indisputable, but to focus on that alone is to miss the wood for the trees.

A final quote from Sun…

“Climate scientists are subject to pressure and difficulty in communicating to the public about global warming science. Overall, I think we have done a good job, but I also sense that the complexity of the dynamic processes seems to be either overlooked or oversimplified in many communications to the public. I suspect that could cause problems down the road, because we know climate can vary strongly on a range of time scales in the complete absence of any external forcing. If you overlook the complex dynamics of the climate system, and you don’t explain those processes up-front to the public, then you can cause confusion down the road.”

With a few unusually cold winters immediately behind us, I would say that mind-numbing confusion has already set in. All the more reason, then, to pay heed to the wisdom of De-Zheng Sun and others who call for a change in approach to climate science and communication.