Storms of My Grandchildren (18 page)

The atmosphere today contains about 800 GtC as carbon dioxide. Plants contain about 600 GtC, primarily the wood in trees. Soils contain about 1,500 GtC, which is mainly humus, decomposed organic matter. There is almost 40,000 GtC dissolved in the ocean.

Large, natural back-and-forth fluxes of carbon pass among these reservoirs. Plants take up carbon dioxide in photosynthesis, but plants and soils rapidly respire a similar amount of carbon dioxide back to the atmosphere. Carbon dioxide dissolves into cold ocean regions, but a similar amount is released to the atmosphere at other places. These uptakes and losses nearly balance over the year, but small imbalances occur and provide important climate feedbacks. For example, in interglacial-to-glacial climate change, as the ocean becomes colder, it dissolves more carbon dioxide, causing the atmosphere and plants to contain less carbon dioxide, which then drives further cooling. Conversely, when Earth’s orbit or the tilt of the spin axis cause melting of snow and ice, this increases absorption of sunlight, and the warming ocean and soil release carbon dioxide and methane. This greenhouse gas amplifying feedback, as I showed earlier, accounts for nearly half the glacial-interglacial global temperature change.

Humans alter this natural carbon cycle in two major ways: by the burning of fossil fuels and deforestation. The rate at which fossil fuel carbon dioxide is injected into the global atmosphere is known with reasonably high accuracy, because oil, gas, and coal are well-tracked international commodities. The error in annual global fossil fuel use is probably less than 10 percent, even though some governments may not accurately report internal coal uses or sales.

Deforestation is the second important human-made source of atmospheric carbon dioxide. However, the magnitude of the annual deforestation rate is not known accurately. Valuable insight into carbon cycle uncertainties is provided by the simple ratio of the two quantities that are well known: the annual increase of atmospheric carbon dioxide divided by the fossil fuel emissions in the same year. This “airborne fraction” is the dashed curve in figure 16, shown in percent on the right-hand scale.

Remarkably, the airborne fraction, averaged over several years, has been nearly constant for fifty years at an average value of only 56 percent. In other words, even if we assume that there is no net deforestation, 44 percent of fossil fuel carbon dioxide is disappearing into sinks. Sinks are places—such as the ocean, forests, and soils—that can take up some of the excess carbon that humans are putting into the air. It is fortunate that sinks have been able to remove a significant fraction of the human emissions—otherwise the climate change would be larger. Recently, on the basis of both models and observations, the ocean is estimated to be taking up about 3 GtC per year. Thus, given the fossil fuel source of 8.5 GtC per year and the average atmospheric increase of 4.5 GtC, the total sink must be 4 GtC per year. Given the estimate of 3 GtC per year for the ocean sink, all other factors—mainly vegetation and soils—together must produce a net sink of about 1 GtC per year.

The fact that Earth’s land masses continue to produce a net sink of carbon dioxide provides a glimmer of hope for the task of stabilizing climate. This carbon sink occurs despite large-scale deforestation in many parts of the world, as well as agricultural practices that tend to release soil carbon to the atmosphere. Improved agricultural and forestry practices could significantly increase the uptake of carbon dioxide, as we’ll see later.

Any optimism, however, is dependent on the assumption that fossil fuel emissions will decline. If, instead, emissions continue to increase, the terrestrial system may become a less effective sink or even become a source of greenhouse gases. Some climate models predict, for example, that continued global warming will cause drought and forest fires in the Amazon, turning that region into a large source of carbon dioxide.

It follows that the world, humanity, has reached a fork in the road; we are faced with a choice of potential paths for the future. One path has global fossil fuel emissions declining at a pace, dictated by what the science is telling us, that defuses amplifying feedbacks and stabilizes climate. The other path is more or less business as usual, in which case amplifying feedbacks are expected to come into play and climate change will begin to spin out of our control.

Well, then, how can we evaluate as precisely as possible the path that humanity is beginning to travel at this critical time? One way is to keep our eyes on two key numbers with respect to the carbon cycle.

The first key number is the rate at which carbon dioxide is being pumped into the air by fossil fuel burning. This is shown by the solid line in figure 16.

This second key number is precisely known because of the monitoring that Keeling initiated. However, I must warn you that the value of this quantity fluctuates a lot from year to year, as shown in figure 17, so do not take observations in a single year as a basis for either alarm or rejoicing. One reason for the fluctuations is the oscillation of ocean surface temperature associated with the El Niño–La Niña cycle, which affects the ocean’s ability to absorb carbon dioxide. Also, droughts reduce the ability of vegetation to take up carbon dioxide, and forest fires release carbon dioxide.

The five-year mean is included in figure 17 to minimize the effect of fluctuations, but even this mean is quite variable. The largest fluctuation is the slow growth of atmospheric carbon dioxide in the early 1990s, which is probably an effect of the massive eruption of the Mount Pinatubo volcano in 1991. Pinatubo cooled the ocean for a few years, causing the ocean to dissolve more carbon dioxide. Volcanic aerosols also scatter incoming sunlight, making incoming light more diffuse (causing the sky to appear slightly milky in daytime). Plants grow better, and thus sequester more carbon, if the sunlight is diffuse rather than beating down on them from one direction.

As time goes on, it will become more and more useful to compare observed carbon dioxide growth with scenarios for the future, which are included in figure 17. All scenarios defined by IPCC have carbon dioxide growing faster and faster in the future. We shall see anon that all these scenarios yield climate disaster. Yet these scenarios are consistent with projections of government energy agencies, which universally assume, seemingly as a god-given fact, that all fossil fuels will be burned at a faster and faster rate. In contrast, our alternative scenario, also shown in figure 17, assumes that humanity is capable of exercising free will in determining its energy sources.

Both IPCC scenarios and the alternative scenario were defined in the late 1990s and published in 2001 and 2000, respectively. So now, real-world data from almost a decade are available for comparison with those scenarios. It is apparent that, so far, the world has continued on a business-as-usual path. Thus real-world carbon dioxide growth has exceeded that in our alternative scenario. Yet, as will be shown, it is still feasible to achieve a carbon dioxide amount even lower than that in the alternative scenario. Such a path would require restrictions on emissions from coal and unconventional fossil fuels, such as tar sands.

I hope that the key quantities defining climate change, and its causes and consequences, as summarized in the appendix and updated monthly on my Web site, will help the public understand climate change as it progresses and distinguish reality from propaganda and hyperbole. In that regard, the growth rate of atmospheric carbon dioxide in figure 17 provides a good example. It is apparent that, despite year-to-year fluctuations, carbon dioxide is growing at a rate in good agreement with the IPCC business-as-usual scenarios but faster than the alternative scenario.

This reality contrasts markedly with the impression created by the media. One frequently reads that greenhouse gases including carbon dioxide are increasing more rapidly than expected, emissions are exceeding expectations, ocean sinks have decreased, the soil has become a source of carbon dioxide, or deforestation has increased. A given story may have some basis in reality, but the net result is a misimpression, as figures 16 and 17 make clear.

Sometimes these stories appear simply because the media has a desire to find interesting “news” to report. Albert Einstein had misgivings about scientists describing research progress to the media, especially preliminary results, because inevitably, he said, this creates “the impression that every five minutes there is a revolution in Science, somewhat like a coup d’état in some of the smaller unstable republics.” In addition, climate change has become a political issue, which can color how new observations are reported.

If I do a good job of choosing and explaining the quantities that are needed to define and understand climate change, it may help you as you try to assess the continuing course of Earth’s climate, the forces that drive climate change, and the impacts of climate change. Quantities selected must be not only central to the “physics” of the problem but also measurable with a meaningfully small uncertainty.

In summary, the two key quantities for the carbon cycle are the annual carbon dioxide emissions from fossil fuel burning and the annual increase of atmospheric carbon dioxide. Regarding the first quantity, note in figure 16 that the Kyoto Protocol, which was adopted in 1997 and went into force in 2005 with “legally binding commitments” to reduce greenhouse gas emissions, did not lead to a decrease in global emissions—indeed, emissions continued to increase. The second quantity, annual change of carbon dioxide in the air (figure 17), so far is closely following business-as-usual scenarios, which are based on the assumption that all fossil fuels will be burned. Finally, the ratio of these two key quantities, the dashed curve in figure 16, provides an additional important conclusion: The net “sink” for human-made carbon dioxide is not decreasing; rather, the sink is increasing, as it continues to average about 44 percent of emissions, which are increasing.