Guest essay by Andy May
This version corrects an error in the average concentration of water vapor in the atmosphere.
The Earth’s dry atmosphere is 78% nitrogen, 21% oxygen and 0.9% argon. These are not greenhouse gases and they total 99.9%, leaving little space for the greenhouse gases methane, carbon dioxide and water vapor. The amount of water vapor in the atmosphere varies a lot with altitude and temperature. At low altitude and high temperatures (greater than 30°C or 86°F), over the ocean, it can reach 4.3% or more of the atmosphere and is less dense than dry air, causing it to rise. It will rise until the temperature is low enough for it to condense to a liquid or solid state and form clouds, rain or snow.
The amount of water vapor in the air drops to very close to zero when temperatures are below -10.0°C. Thus, the average volume of water vapor in the total atmosphere is variable and usually between 1% and 2%. So, excluding nitrogen, oxygen, water vapor and argon, we are left with 0.1% for everything else. Water vapor is a powerful radiative greenhouse gas and where the concentration is high, over the tropical oceans, it has a large radiative greenhouse effect. But, over land and in the cooler high latitudes, there is not enough of it to have a significant effect.
Carbon dioxide makes up 0.04% of the Earth’s atmosphere and is more evenly distributed than water vapor. Nearly all of the radiative greenhouse effect in dryer areas is due to carbon dioxide, with a small contribution from methane. Methane makes up about 0.00018% of the atmosphere on average, it is distributed unevenly like water vapor. Over swampy areas with a lot of vegetation or over farms it can be high. Over most of the Earth it is essentially zero. Methane is very reactive and is removed from the atmosphere quickly after it is released.
Carbon dioxide, water vapor and methane are the main radiative greenhouse trace gases in the atmosphere. There are some other trace gases, like neon, krypton and xenon, but they are not greenhouse gases. The IPCC (WG1 AR5) likes to add N2O (nitrous oxide or laughing gas) to the greenhouse list. It is a gas emitted from oceans, soils, fertilizer and burning biomass. It is present in the atmosphere in very low concentrations (0.000032%) and is very reactive. It is a rocket fuel and a race car gasoline additive, after all. This volatility results in a very short atmospheric lifetime, so it is hard to understand how it could have much of a greenhouse effect. The gas is so safe it is approved as a food additive and as a propellant for whipped cream! Further, even the IPCC admits on page 468 of WG1 AR5, the added nitrogen increases natural CO2 sinks (basically increases plant growth) so the net effect of nitrous oxide may be to reduce the greenhouse effect. This post will focus on CO2.
Carbon dioxide is emitted when animals and some microbes breathe, from the oceans (which contain 93% of the carbon dioxide on Earth) and when plants or fossil fuels are burned. In the 1990’s fossil fuel emissions were about 3% of the carbon dioxide entering the atmosphere according to the EPA. About half of the fossil fuel emissions were absorbed by the environment. Mostly the CO2 emissions were absorbed by the oceans, land plants, and marine algae. Additional carbon dioxide in the atmosphere is a powerful fertilizer, for a dramatic illustration of the effect, see this short youtube video. Figure 1 shows the impact of additional carbon dioxide on pine trees under controlled conditions. The four CO2 levels tested are, from left to right, 385 ppm, 535 ppm, 685 ppm, and 835 ppm.
Figure 1 (Pine trees grown at ambient CO2 and three higher CO2 concentrations under controlled conditions, source)
Additional carbon dioxide causes plants to produce fewer stomatal pores per unit of leaf area. Stomatal pores (or stomates) are how plants breathe in carbon dioxide and lose water and oxygen to the air. Fewer stomates mean less water loss due to evaporation, lower sensitivity to pollution, and more resistance to heat and cold. There is compelling evidence that the rising carbon dioxide concentration in the atmosphere is a primary cause of observed recent greening of the Earth. Satellite data shows that the Earth is greener now than in the 1980’s by 6 to 13%. Dr. Ranga Myneni (Boston University) estimates a 14% increase in ecosystem productivity in the past 30 years. The IPCC WG1 AR5 Report discusses the CO2 fertilization effect on page 502. They estimate a greening of the Earth, due to warming and CO2, of 6%. This is at the low end of published estimates. On the same page of the report, they say:
“Thus, with high confidence, the CO2 fertilization effect will lead to enhanced NPP [net primary plant productivity], but significant uncertainties remain on the magnitude of this effect…”
Forests have expanded worldwide (Phillips, et al. 1998), due in part, to increased CO2. Figure 2 shows the rainforest growth rates for dryer and wetter areas in Australia’s Kakadu National Park.
Figure 2 (Craig Idso)
Figure 2 is from Banfai and Bowman (2006). Their aerial study of Australia’s Kakadu National Park found that the rainforest expanded 28.8% between 1960 and 2004. This expansion was mainly due to additional rainfall. But, more importantly they found that the dryer areas of the park increased 42%, whereas the wetter areas increased only 13%. The dryer area’s increase was consistent with the overall increase in CO2.
Figure 3 shows the greening of the African Sahel from 1982 to 2003.
Figure 3 (Craig Idso)
The NDVI values mapped in figure 3 are the Normalized Difference Vegetation Index. It was computed by Herrmann, et al. (2005) and they found that:
“…rainfall emerges as the dominant causative factor in the dynamics of vegetation greenness in the Sahel … [but] the vegetation greenness [was] beyond what would be expected from the recovery of rainfall conditions alone.”
The warming we have seen over the last 130 years or so has also had a positive effect on plant life. The warming of the planet allows plants to move farther north and south, expanding the vegetated area. The fact that plants grow more efficiently, are more resistant to temperature extremes, and with less water; allows them to encroach into areas that were previously unproductive deserts. There are numerous peer-reviewed studies supporting the greening of the Earth due to increasing carbon dioxide and warmer temperatures. You can find a good bibliography and a discussion of the literature in Dr. Craig Idso’s excellent online book The State of Earth’s Terrestrial Biosphere. Figure 4 illustrates the effect of additional CO2 and warmer temperatures on big tooth aspen leaves.
In figure 4 notice that the peak productivity temperature increases from 25°C to 36°C at the higher CO2 concentration. As noted above, the additional CO2 means fewer stomates in the leaf and a greater resistance to drought and temperature extremes. One of the reasons global warming is unlikely to reduce plant productivity is that plant productivity rises with increased CO2 and the optimum productivity temperature increases as well. One critical fact that Dr. Idso discusses is that:
“Earth’s land surfaces were a net source of CO2-carbon to the atmosphere until about 1940. From 1940 onward, however, the terrestrial biosphere has become, in the mean, an increasingly greater sink for CO2-carbon. Over the past 50 years, global carbon uptake has doubled from 2.4±0.8 billion tons in 1960 to 5.0±0.9 billion tons in 2010.”
This is also acknowledged by the IPCC in WG1 AR5 on page 486, table 6.1. But, the IPCC reports much lower figures (2.7 billion tons for 2011) for the land biota CO2 sink. Even the lower number is important because it shows that CO2 absorption, by plants, is increasing as the available CO2 increases. As the book and the references show, the greening of the planet is due, in part, to increasing CO2 and warmer temperatures. In fact, it can be shown that increased CO2 and global warming have benefited the world and mankind. This net benefit may continue until 2080, even using the worst-case IPCC global warming scenario.
According to the IPCC WG1 AR5 Report (page 472), on average, CO2 molecules are exchanged between the atmosphere and surface every few years. Only a tiny fraction (2%) of the total CO2 on Earth is in the atmosphere. Most is stored in the oceans (93%) or in the soil and land plants (5%). So, the fact that both the land and the oceans are now absorbing more CO2 than in the past is important. Perhaps land and ocean uptake of CO2 will slow in the future, as the IPCC suggests, but this has not been demonstrated or observed to date except when vital nutrients are in short supply.
The IPCC WG1 AR5 later declares, alarmingly, on the same page (472), that removal of all human-emitted CO2 will take a few hundred thousand years. This assertion is only supported by geological evidence from the Paleocene-Eocene Thermal Maximum event 55 million years ago, when the average surface temperature was 22°C and the average CO2 atmospheric concentration was 800-1000 ppm. Their logic is convoluted and difficult to follow, but they seem to be saying that the CO2 is not gone until it is taken up by rocks. They completely ignore the likely increase in ocean and land biota that will occur due to extra CO2. Further, why do they assume that carbon dioxide removed from the air and stored in plants is still a problem? This is not explained and I find their arguments unconvincing.
In conclusion, additional CO2 has benefits that have not been fully considered by the IPCC. In doing any analysis of the impact of global warming due to man’s fossil fuel emissions, both the estimated benefits of additional CO2 and the estimated problems need to be accounted for. CO2 is a greenhouse gas, but its net effect on global temperatures and mankind is unknown. CO2 has increased at a much higher rate in this century than in the latter part of the previous century. So far this century it has increased 2.1 ppm/year versus 1.5 ppm/year previously (see the slopes in figure 5, which are in ppm/year) yet temperatures have risen more slowly in this century (.0165°C/year now versus .0188 °C/year previously, figure 6). Figure 5 plots the yearly average carbon dioxide readings from the Mauna Loa Observatory, data is from NOAA here.
Figure 5 (source of data)
Figure 6 shows the yearly HADCrut 4.5 global ensemble median temperature anomalies, the data is from here. I deliberately included 2016 through October to capture the recent El Nino, since the trend for the previous century includes the 1998 El Nino. One must ask, if CO2 is the dominant driver of global warming, why does the rate of temperature increase go down when the rate of CO2 increase is rising?
Figure 6 (source of data)
Comparing two back-to-back 17-year trends is not, strictly speaking, comparing two climatic trends, the period is too short. But, there is certainly no compelling reason to spend billions of dollars in a futile attempt to reduce our carbon dioxide emissions based on the evidence before us today.