Could Doubling CO2 Increase Earth’s Temperature by 9 Degrees C?

by Emil Morhardt

That is the assertion of a paper in Nature by Carolyn W. Snyder (Snyder, 2016) based on an analysis of the correlation between atmospheric CO2 concentrations and changes in the global average surface temperature (GAST) over the past 800,000 years. Actually the assertion is that the 95% “credible interval” is 7 to 13 degrees Celsius (12.6 to 23 degrees Fahrenheit) Yikes! Even the current scientific consensus value of something on the order of 3 C (5.4 F) (Collins et al., 2013) is frightening when you consider that the difference in the GAST between the last glacial maximum about 20,000 years ago and at present wasn’t much different than that. Continue reading

Do Plants Prevent Atmospheric CO2 Levels from Falling Too Far?

by Emil Morhardt

A recent paper discussed in the previous post (Galbraith and Eggleston, 2017) claims that during the past 800,000 years when the Earth has been in a glacial condition with the occasional interglacial period (such as now), there is a strong correlation between global temperature and atmospheric CO2 levels, and that they tend to go to the same low point again and again and stay there. These authors argue that if CO2 were to go lower, so would the temperature, and that therefore something is keeping the CO2 level from going any lower then 190 ppm. One intriguing possibility they bring up comes from a paper (Pagani et al., 2009) by Mark Pagani at Yale, and his colleagues at the Carnegie Institution in Stanford and at the University of Sheffield who claim that plants stop effective photosynthesis if CO2 levels fall below 190 ppm, depriving the carbon cycle of two sources of removal of atmospheric CO2; photosynthesis, and a more subtle plant activity called biologically enhanced silicate chemical weathering. The mechanisms of these two processes are interesting. Continue reading

How Low Does CO2 Go?

by Emil Morhardt

Atmospheric CO2 levels are always lower during glacial periods than during interglacials like the one we are in now. During the last glacial maximum 20,000 years ago, for example, they were at 190 parts per million (ppm), whereas during the most recent 10,000 years, almost up to the present, they have been about 280 ppm. [We have now succeeded in raising them to over 400 ppm and still counting, but that’s a different story.] Eric Galbraith and S. Eggleston (Galbraith and Eggleston, 2017) argue that as far as we know, atmospheric CO2 levels have never gone below the typical glacial levels of 190 ppm, even in extended snowball earth conditions. Why not? Well, a carefully-reasoned 2009 paper they cite (Pagani et al., 2009) suggests that even in the mostly warm conditions of the last 24 million years, when CO2 levels fell below 190 ppm, terrestrial plants stopped effectively photosynthesizing, thus they not only stopped removing CO2 from the atmosphere directly, but they also stopped the active root growth which increases the acidity of soils and enhances chemical silicate weathering from the rocks which removes CO2 from the soil, and ultimately from the atmosphere. Galbraith and Eggleston argue that the same thing has been happening during the glacial periods of the last 800,000 years, and extend the argument to the photosynthesis of oceanic phytoplankton. To wit, when CO2 levels get below 190 ppm, CO2 removal from the atmosphere by photosynthesis and chemical weathering is sharply reduced, so they decline no further. Continue reading

The First Snowball Earth?

by Emil Morhardt

When did the first ice on Earth occur? About the only way to find out is to find old rocks with evidence of glaciation then determine exactly how old they are. A type of rock characteristic of glaciation is diamictite, a conglomerate-like mix of rocks with a large range of sizes held together with clay or mud that has been metamorphosed into mudstone. The large range of intermixed sizes in these deposits indicates lack of the size sorting that occurs in a river bed or floodplain, so some other source of disruption must have occurred, one of which is being scraped off and bulldozed along by a glacier. David Zakharov at the University of Oregon with a team of scientists from around the world (Zakharov et al., 2017) reasoned that if they could find examples of this type of rock that had formed near the equator, and could demonstrate that the water the rock encountered during formation came from glacial meltwater, then, they would have proven that at the time there were glaciers near the equator.  Continue reading

Is There a Future Tipping Point in the Atlantic Meridional Thermohaline Circulation (AMOC)?

by Emil Morhardt

The AMOC is the set of ocean currents that begins with cold seawater off Greenland sinking to the bottom and flowing south, being replaced by warmer water flowing at the surface past Florida from the south, transferring warmth from the tropics to the east cost of North America and the West Coast of Europe. The full round trip cycle from, and back to, Greenland takes a thousand years. Without it, both western Europe and eastern North America would cool significantly, with large numbers of potential adverse effects. We know from ice core record temperature data from both the Greenland Ice Sheet (GrIS) and from Antarctic ice sheets, that the AMOC has come to an abrupt halt many times, and has characteristically taken a millennium to recover. The comparative abruptness of these cessations has led to the fear that there may be some threshold that once crossed—a tipping point—that cessation is inevitable. This would be nice to avoid. Continue reading

Any Reason to Expect a Tipping Point with Arctic Sea Ice?

by Emil Morhardt

Williamson et al. (2016) examined the satellite data looking for signs of a tipping point in Arctic sea ice loss, but found none (my Jan 1 post). About the same time, Notz and Stroeve (2016) looked at the same data and did a simple linear correlation between September Arctic sea ice area and cumulative CO2 emissions since 1850. Voila! There was a strong negative linear correlation between the two showing a sustained loss of 3 ± 0.3 square meters of September sea ice area per cumulative metric ton of CO2 emission. Their title summarizes the result clearly: Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. If this linear trend continues and there is no tipping point—and there is no reason to expect one—we can make a pretty good guess about the timing of the future of Arctic sea ice to the extent we can predict CO2 emission levels. At the rate we are going, September Arctic sea ice will be completely gone before mid-century (and global average temperatures will have risen more that 1.5ºC.) Furthermore, we can now get a feeling for how much our personal use of fossil fuels and the energy derived from the directly affects Arctic sea ice; the average CO2 release from personal use is several metric tons,  Continue reading

Are There Early Warning Signals of Tipping Points in Arctic Sea Ice?

by Emil Morhardt

Everyone agrees that it would be helpful in making climate policy if we had some advance warning of climate tipping points. Williamson et al. (2016) set out to look for one associated with the melting of the Arctic sea ice. They asked if there is any evidence that the sea ice is about to undergo a local tipping point (which they also refer to as a bifurcation) that would lead to much faster melting. In the case of Arctic sea ice, the annual melting is caused by the summer sun, and a long-term parameter change that is enhancing it is the gradual buildup of CO2 in the atmosphere. Melting is limited partially by the fact that much of the incoming solar radiation is reflected back to space by the ice. A tipping point might be reached when enough of the ice is melted that the amount of sunlight absorbed by the ice-free open ocean begins to warm it faster each cycle. Although this study did not find any evidence for an imminent tipping point the authors noted that the sort of signal they were looking for might not occur until very close to a tipping point. That would, of course, make it useless for long-term policymaking. Their approach to looking for a signal is interesting though, and could be applied to many other potential climate tipping points. Continue reading