Constructed Climates

Figure 3.6: The left plot extends the recent CO₂ concentration plot to include the last two centuries and shows emissions from fossil fuel, cement, and gas flare sources combined (data from Marland et al. 2008). The steep change in the two curves’ slopes as they shoot upward take place remarkably close in time, and total emissions roughly equal the total increase in atmospheric carbon. Shown at right are CO₂ levels (the lines) from the last 400,000 years, along with corresponding temperatures (the squares), measured from air bubbles in 3.6 km deep glacial ice cores from Vostok station in the eastern Antarctic (after Petit et al. 1999). Variations in CO₂ levels and temperature clearly reflect one another, and our recent fossil fuel CO₂ emissions likely signal future (and present) temperature increases.

Simple facts include that the world is round, the Earth orbits the Sun, natural selection drives evolution, and CO₂ levels are incredibly high. Despite sunlight-driven photosynthesis absorbing lots of CO₂, atmospheric CO₂ levels clearly and undeniably increased over the last 200 years. I show the recent trend in the left graph of Figure 3.6. When I was born in 1960, the level was about 320 ppmV (parts per million by volume), and about 50 years later the levels are more than 380 ppmV, nearly 20% higher. For convenience, I’ve left space on the right-hand side of the graph to fill in the curve over the next four decades. Scientists don’t like projecting curves very far beyond their data: What if an asteroid hits? Or a mega-volcano erupts? Or the oceanic currents change? Or the frozen tundra emits more carbon than estimated when it thaws? Many things could cause CO₂ predictions to be off, but, still, just guessing here, CO₂ levels are likely to be somewhere around 460 ppmV when my children reach my age today.

People cause rising CO₂ concentrations. A rather striking correlation, also shown in the left-hand plot, exists between the very recent, very rapid rise in CO₂ and the simultaneous, very recent, very rapid increase in CO₂ emissions by people.[28]

It’s also a simple fact that temperature and CO₂ levels show a high correlation over a very long time, as the graph on the right shows. The lines depict CO₂ levels, and the data points indicate temperature — it’s not simply a matter of lines connecting points, they’re two different things! Physical processes tightly correlate the two, a correlation much stronger than, say, asthma and air pollutants (see Figure 6.4).

Again, these graphs show that present CO₂ levels are passing through 380 parts per million, compared to the highest historical level over the last 400,000 years, which saw a maximum of about 300 ppm.[29] Since then, we (me included) have sent a great deal of carbon dioxide into the atmosphere. A carbon atom spends about four to five years in the atmosphere before some plant fixes it into its tissue or it gets mixed into ocean water.[30] If we could just stop emitting all fossil carbon, estimates show that the biosphere could pull atmospheric CO₂ levels back toward “equilibrium” in about 50 to 200 years.[31]

Are humans responsible for increasing CO₂, or is it just “natural” variation having nothing to do with humans?[32] One study examined this long time series of data, identifying the time scales present in CO₂ changes. Important scales are daily (night versus day levels), annual (summer versus winter levels),[33] and glacial scales of around 100,000 years.[34] Besides these, the last 100 years emerge from background CO₂ variation as a unique and strong peak, which gives strength to the argument that the last 100 years are due to humans.[35]

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[28]Six gigatons, the recent level of carbon emissions, equals 6 × 10⁹ tons, or 12 × 10¹² pounds, or 5 × 10¹² kg. The atmosphere contains about 750 trillion kg of carbon, so emissions seem like a small fraction. However, Figure 3.6 shows that since 1950, the atmospheric concentration increased by about 20%, or about 150 trillion kg. Average annual emissions of around, say, 2 × 10¹² kg for a period of 60 years totals about 120 trillion kg, a pretty close number.

[29]Of course, these historical CO₂ values have a coarse-grain measure over about 1,500 years, certainly longer time scales than we’ve been measuring over the last few decades. This results in something like comparing the EPA’s 8-hour ozone levels versus 1-hour ozone levels. However, it seems tenuous to use that fact to comfort oneself that today’s levels are just “natural” cycles, having nothing to do with the enormous amounts of fossil carbon we’ve released back from sequestration.

[30]Residence time of carbon atoms in the atmosphere is obtained by dividing the amount of carbon in the atmosphere, 750 trillion kg, by the amount absorbed by plants and oceans each year, roughly 200 trillion kg. That gives about four years. Another way to look at these numbers: If all the atmospheric carbon fell on the ground, there’d be about 1.5 kg/m², of which human contributions total about 0.4 kg/m², or about a pound per square yard. Yearly fossil fuel emissions into the atmosphere amount to 16 grams of carbon per square meter.

[31]Here’s a rough calculation. If we could stop emitting all fossil fuels, the balance of all the carbon transfers would sum up such that around 3 trillion kg would be pulled out of the atmosphere each year. Dividing a rough estimate of the excess carbon in today’s atmosphere, perhaps 175 trillion kg, by that sum gives 175/3 = 58 years. That’s if all fossil carbon emissions stop.

[32]Some might argue that since humans have been around for the entire 400,000 years that the long-term graph covers, clearly we can survive varying CO2 levels and temperatures. What’s another few hundred thousand years, and if we already see such large natural variation, why worry? I believe those arguments are irrelevant to the current situation: Our present high population depends critically on growing food where we presently grow food. Rapid climate change could disrupt food production and imperil many people. Human-caused climate change, including global warming, is true, independent of whether one chooses to believe it to be so, but this topic falls outside the purview of a book on urban environments.

[33]Land plants take in lots of CO2 from the atmosphere, and the preponderance of the northern hemisphere’s land mass makes it’s seasonal plant growth observable.

[34]Glacial cycles in temperature and CO2 result from something called Milankovitch cycles (Shackleton 2000). The Earth’s orbit around the Sun isn’t a simple circle; rather it has many ways to be different from a static, pure circle. I always fall back to thinking of a child on a swing. The swing is a pendulum with a “preferred” frequency. The child can pump the swing too fast, too slow, or just right to force the swing to swing. Now imagine a line of 10 swings of different lengths made out of different kinds of ropes and chains, and the swings are linked together one to the next by rubber cords. Ten children pump these swings and out pops an overall swinging of the swings. In a similar way, all of the variability in Earth’s orbital motions, put all together, leads to climate cycles on Earth having 100,000-year cycles.

[35]Falkowski et al. (2000) present the analysis of CO2 periods in historical records.