Unanswered Climate Change Questions, Answered

In a recent article for the Wall Street Journal, John Steele Gordon asserts that climate science is a "veritable conucopia of unanswered questions". Gordon offers a sample of these questions:

Why did the warming trend between 1978 and 1998 cease, although computer climate models predict steady warming? How sensitive is the climate to increased carbon-dioxide levels? What feedback mechanisms are there that would increase or decrease that sensitivity? Why did episodes of high carbon-dioxide levels in the atmosphere earlier in Earth’s history have temperature levels both above and below the average?

Maybe Wall Street Journal staff and contributors don't have access to the most recent climate resesarch, but that's no reason to assume the research has not been done or the results have not been published. In reality, all of these questions have been answered long ago.

Warming since 1998

The answer to this one is a little complicated; first of all, it's a fact that 1998 set a record that stood for most of a decade, and it's also a fact that if you use 1998 as the midpoint, the total warming in the 20 years prior is more than the total warming in the 17 years since. However, 1998 was a very unusual year. Due to an extremely strong El Niño, 1998 was much warmer than any previous year on record. Likewise, 1978 was an unusually cool year. The following chart shows average yearly temperature anomalies since humans started recording temperatures.

It's not hard to pick out 1998 (the peak just prior to the year 2000) and 1978 (the valley right before 1980). One might conclude these two endpoints were cherry-picked in an attempt to make the case that warming is a thing of the past.

But when we look at the big picture, we can see a continuing warming trend; in fact, we've now had four years (2005, 2010, 2013, and 2014) warmer than 1998, with 2014 the warmest yet. And we're on pace to set a new record again in 2015.

Cherry picking simply does not make for good science. We could just as easily have cherry picked 1980 to 1996 vs. 1996 to the present, and asked why global warming didn't exist before 1996. But the reality is, when we look at the big picture, global warming is a threat; it's been with us for four decades now, and it is not going away.

CO₂ sensitivity

The 1979 Charney Report estimated that a doubling of atmospheric CO₂ would increase global temperatures by 3° Celsius, ± 1.5°, giving us a minimum of 1.5° and a maximum of 4.5°. A 2006 paper by Annan and Hargreaves tightened the range a little, giving a minimum of 2°, a maximum of 4.5°, and a best estimate of 2.9°. While this is not as precise as we might like, we can at least put it in perspective: The total warming since we've started measuring has been less than 1° C. While we might desire a bit more certainty than science can currently deliver, we can be assured that a doubling of atmospheric CO₂ levels will bring more global warming than we've ever seen.

Furthermore, we know that CO₂ already has a greater impact on the climate than any other component.

Of all the ways to keep global warming in check, reducing CO₂ emissions would have the greatest impact.

Feedback mechanisms

Several feedback mechanisms are known and understood. One of the best-known involves water vapor. Higher levels of water vapor in the atmosphere cause the air to retain more heat, and warmer air can hold higher levels of water vapor, which causes the air to retain even more heat. The ice-albedo feedback is also well understood. Albedo is the reflectivity of the earth's surface. Increase the amount of ice cover, and more sunlight (and heat) will bounce back into space, cooling the atmosphere, which will lead to more ice cover. Conversely, decrease the amout of ice cover, and less heat will be reflected back into space, warming the atmosphere and melting more ice.

But the more we understand about feedback loops, the more complicated it gets. Chris Colose explains in a guest post for RealClimate.org:

When multiple feedbacks operate, they can add together in rather odd ways. For instance, you might think that if you take a feedback that doubles the sensitivity to climate and another that halves it, they would cancel and bring you right back to the no-feedback sensitivity. You might also think that two feedbacks which each amplify the original forcing by 50% would add to double the no-feedback sensitivity. In fact, neither of these is the case, and the behavior emerges because multiple feedbacks interact with each other as well. One can imagine that if water vapor and the ice-albedo feedback are operating, the water vapor boost will mean more ice melt, which will mean further warming, more water vapor, still less ice, and so forth.

Other feedback mechanisms can be even more complex. Clouds, for example, increase the earth's albedo, allowing less heat from the sun to reach the earth. But clouds also trap heat below them, preventing it from escaping into space. The net temperature effect could be positive or negative, depending on the type and the height of the cloud.

To understand how all the inputs and feedbacks interplay and affect the climate, scientists build climate models. By adding (or subtracting) the net effect of all the known influences on our climate, and comparing the result to actual recorded temperatures, they can determine how accurate the model is. Their understand of our climate is now good enough to build models that account for all of the net warming since we first started recording temperatures.

There are still uncertainties involved in climate modeling. For one thing, we cannot predict sporadic large-scale events like volcanic eruptions or El Niño cycles. But if—after all known inputs and events are accounted for—the model approximates our observations, we can be reasonably certain we aren't missing anything critical.

High CO₂ levels in history

It's no secret that CO₂ levels in the distant past have been much higher than they are today. There was even apparently an ice age during the late Ordovician period with CO₂ levels between 4000 and 5000 ppm, an order of magnitude above today's levels. Global warming deniers like to trumpet this fact as if it singlehandedly contradicts the role of carbon dioxide in climate change.

But as we've already seen, many factors are involved in determining how much heat the climate retains. For one, we understand how a star's output grows over time. 450 million years ago, during the Ordovician, the sun's output was about 4% less than it is today. It doesn't sound like much of a difference, but it's enough to give us an ice age at 3000 ppm of CO₂.

Furthermore, thick layers of bentonite—rock formed from weathered volcanic ash—indicate that the late Ordovician was a time of heightened volcanic activity. Volcanic eruptions spew a number of gases into the air. CO₂ is one of them; SO₂ (sulfur dioxide) is another. Sulfur combines with water vapor in the upper atmosphere to form natural aerosols, which are known to have a cooling effect. Volcanoes also produce massive amounts of ash, which can prevent sunlight from reaching the earth.

During the Ordovician, the bulk of the world's land mass was joined in the supercontinent of Gondwana, which at the time was located over the South Pole. The high level of water vapor in the atmosphere, due to the high CO₂ levels, made conditions right for heavy precipitation. A slight drop in the earth's temperature could have turned that precipitation into snow, which would have formed glaciers in Gondwana. And from what we know about feedback loops, glaciers would lead to lower temperatures, and more glaciers. Under those conditions it would only take a modest dip in CO₂ to trigger a brief ice age. And the Ordovician ice age was brief, lasting perhaps a half million years. By contrast, the previous ice age, during the aptly-named Cryogenian period, lasted around 100 million years; the Huronian glaciation, more than two billion years ago, lasted 300 million years. These ice ages were so long and so extensive that some researchers have postulated the "snowball Earth" hypothesis, which claims that at these times the planet's entire surface was frozen. Compared to these, the Ordovician ice age was just a blip.

But none of that is relevant to today. Although the climate is very complex and is affected by many factors, we know that an increase in CO₂ from fossil fuels is the primary driver of the current warming trend. Too many lines of evidence point in the same direction; the only conclusion that fits is that today's warming, unlike historical climate change, is caused by human activity. That fact can't be changed by pretending we don't know it.