Writing about vaccines, evolution, and even dark matter has ended up setting off contentious discussions here at Ars. But no area seems to bring out impassioned arguments as reliably as climate change. Covering the latest scientific results can bring forth cries of scientific fraud, conspiracies, and denialism; considering policy implications can be even worse.

It can be really difficult for anyone not well-versed in the debate to get any sense of the science at all, something that's clear from the huge gap between the scientific community's acceptance of climate change and the public's wariness about the topic. So it's probably useful to step back from the latest findings, and look at science's basic understanding of how greenhouse gasses can force climate change, which often gets lost in the arguments.

All things being equal, adding greenhouse gasses to the atmosphere will warm it

The concept of greenhouse gas-driven warming was worked out about a century ago, shortly after it was realized that carbon dioxide is transparent to visible light, but absorbs infrared radiation. Once absorbed, it is transformed into vibrational and rotational energy, which we perceive as warmth. This plays out very simply: the sun's output in the visible spectrum passes through greenhouse gasses on the way to the ground, where a lot of it gets absorbed and radiated back out as infrared at a later point. With more greenhouse gasses in the atmosphere, a greater fraction of that IR is absorbed by the atmosphere, increasing its heat content.

Not only are the physics simple, it's easy to calculate the impact of greenhouse gasses by measuring the amount of energy sent our way by the sun, and then assuming it's all radiated back to space. Without the influence of greenhouse gasses, including water vapor and CO 2 , the planet would be a much colder place, and uninhabitable by most of its current life.

The basics of the greenhouse effect, courtesy of the Department of Energy

It's also relatively simple to calculate that adding more of these gasses would shift the energy balance further. This really can't be emphasized enough. In the hyperbolic language that has infested the debate, researchers have been accused of everything from ditching the scientific method to participating in a vast conspiracy. But the basic concepts of the greenhouse effect is a matter of simple physics and chemistry, and have been part of the scientific dialog for roughly a century.

Greenhouse-driven climate change is used to explain many historic events

There are any number of indications that climate change driven by greenhouse gasses is far more readily accepted among the scientific community than it is among the general public, from surveys of researchers to statements released by organizations of professional scientists. But perhaps the clearest indication of the acceptance of an idea like this is the fact that it's deeply engrained in scientific discussions. The scientific literature makes it clear that scientists view greenhouse effects as plausible explanations for everything from historic shifts in the Earth's climate to the dynamics of the atmospheres of other planets.

In the last month or so, I've come across a paper that suggested the global glaciations occurred when oxygen appeared in the atmosphere because that oxygen reacted with and eliminated a specific greenhouse gas (I'd seen something similar years ago that proposed the same thing, but with a different greenhouse gas). In the same span, two papers have suggested that a plunge in atmospheric carbon dioxide caused the glaciation of Antarctica. The importance of atmospheric carbon dioxide for an earlier period of warming called the PETM has also been an area of intensive study.

It's important to emphasize that, with the exception of the PETM, the precise role of greenhouse gas changes in these events is pretty speculative at this point. But a couple of things should be clear: scientists consider changes in greenhouse gasses a plausible driver of changing climates, and view the field as a fruitful area of study.

That "all things being equal" caveat is a big one

Obviously, however, greenhouse gasses are hardly the only climate influence that can change. The amount of energy sent our way by the sun varies in 11-year cycles, and often displays longer-term trends. Aerosols, such as those released by volcanoes and burning fossil fuels, can reflect sunlight back into space; so can some clouds.

The levels of any of these can change at the same time as greenhouse gasses, either enhancing or obscuring their climactic impact. As a result, researchers have attempted to track the changes in these other factors in order to understand the precise role played by rising levels of CO 2 and methane in recent temperature rises. These efforts have included tracking the activity of the sun (pretty easy) to measuring the levels of aerosols and the extent of cloud cover (much, much harder).

Complicating matters further, a change in temperature can induce changes in other factors that influence climate. So, for example, higher temperatures melt glaciers, which normally reflect sunlight back into space—their absence will tend to enhance the warming. In the same way, higher temperatures allow the atmosphere to hold more water vapor, which is a potent greenhouse gas (but one that circulates through the atmosphere within days, rather than the centuries typical of CO 2 ).

It's pretty clear that these feedbacks play a critical role in modulating the climate—the Earth wouldn't either enter or exit ice ages if it weren't for feedbacks that enhance otherwise minor changes in the distribution of sunlight that reaches the planet. That said, the relative importance or magnitude of different feedbacks is an active area of research, and there is plenty of room for scientific disagreements there.

Still, not knowing everything isn't the same as not knowing anything; it's possible to get some reasonably accurate estimates of many of the feedbacks and forcings, or to set upper and lower bounds on their likely values. In the case of solar forcings, we actually have pretty good measurements of the changes in our sun's activity over the last several decades. The limits we have on these values mean that future estimates, such as those provided by the IPCC, include a range of potential values for likely future temperature changes.

This isn't weather forecasting

Both sides of the public debate have a really unfortunate tendency to equate climate with the weather. People will happily point to extreme weather events as evidence of a changing climate, even though the links between increased temperatures and, say, hurricane strength, are still not entirely clear. Others will list news reports of record low temperatures, apparently oblivious to the fact that one unusually cold day can easily occur in an exceptionally hot year.

These arguments conflate two very different things: weather is what happens on a given day, while the climate is what the weather will typically be like. To give a simple example: the climate in Texas is generally warmer than that in New York, but it's entirely possible to have a colder day in Texas, or hotter day in New York. So, although the global climate is now warmer than it was in 1900, that certainly doesn't preclude the possibility of record cold days.



Image ?UCAR, graphic by Mike Shibao. The difference between weather and climate in the US. Record highs and lows (weather) continue to occur, but the climate drives decade-long trends in their frequency.

There's a related misconception: many seem to think that if we can't get weather forecasts right, we can't possibly predict the climate. Not surprisingly, if weather and climate are very different things, the methods and tools used to predict them are equally different. We've gone into the differences in detail in the past, but the difference is what you'd expect based on the difference between weather and climate: it's very hard to predict the specific state of a complex system over longer time periods, but nowhere near as difficult to get a sense of what its general outlines are likely to be.

It's important to look for the science

It's impossible to cover every possible aspect of climate science in a single piece; the climate is big and complex, and raging arguments have developed over all sorts of minutia. So, the best piece of advice one can have when attempting to engage with climate science is simply to recognize scientific data and reasoning.

So, using examples from above, if you see someone confusing weather and climate, you can probably assume that they've not been paying careful attention to scientific issues. In the same way, you'll often see people arguing that since the planet's climate has changed in the past without human intervention, there's no reason to worry about human-generated greenhouse gasses now—which ignores evidence indicating some of those past changes have been driven by greenhouse gasses. We've also discussed (twice!) why using 1998 as a starting point for discussing some sort of "global cooling" involves a careful preselection of data.

You'll also see some people leave basic logic behind. Many will cite surfacestations.org in an attempt to show that recent temperature records are inaccurate, and then turn around and use those same records to argue that temperatures have recently cooled.

So, in addition to knowing the basics of climate science, perhaps the most important thing to know when dealing with the climate is how to recognize when someone has stopped using science entirely.