Will the Anthropocene last for only 100 years?

On November 9, 2013, methane levels as high as 2662 ppb (parts per billion) were recorded, as indicated by the red dot on the image below.

This image, from an earlier post, gives an idea of the height of this level compared to historic methane levels, and how fast levels of methane (CH4) have been rising compared to levels of two other greenhouse gases, i.e. carbon dioxide (CO2) and nitrous oxide (N2O).

CO2 concentrations in the atmosphere have now risen to levels well above the 400 parts per million (ppm), as illustrated by the graph below, from keelingcurve.ucsd.edu. This 400 ppm is 143% the pre-industrial peak level of 280 ppm.

Paleorecords show that greenhouse gases levels go up and down in lockstep with temperatures in history. The image below shows that carbon dioxide levels back in history typically moved between approximately 180 ppm and 280 ppm, a difference of 100 ppm. Since 1950, CO2 levels have risen by roughly the same difference.

In a fascinating lecture, Dr Jan Zalasiewicz suggests that the Anthropocene started around 1950, when levels of greenhouse gases started to rise exponentially, in line with the rise of fossil fuel use, as also illustrated by the image below.

The image below, from an earlier post, shows that temperatures typically moved up and down by roughly 10 degrees Celsius between a glacial and interglacial phase of the ice ages, suggesting that a 100 ppm rise of carbon dioxide and 300 ppb rise of methane go hand in hand with a 10°C temperature rise.

Many eminent scientists have warned that the high current carbon dioxide levels have already locked us in for a future temperature rise of several degrees Celsius, a rise that is yet to fully manifest itself and that is only held off by the temporary masking effect of sulfur dioxide that is emitted when burning fuel (especially coal) and by the (decreasing) capacity of oceans, ice sheets and glaciers to act as a buffer for heat. Once the masking effect of sulfur dioxide ends and the Arctic sea ice collapses, a huge sudden rise in temperature can be expected, hitting vulnerable pools (see image below) which would accelerate the temperature rise even more and could cause temperatures to rise by another 10°C within decades.

The scenario of such a huge rise in temperature becomes a distinct possibility when considering the combined warming impact of carbon dioxide, methane, nitrous oxide, water vapor and albedo changes, and the vulnerability of some of the terrestrial and marine carbon pools. Also note that, while the above Unesco image gives an estimate of 10⁴ or 10,000 Gt C for ocean methane hydrates, several studies give even higher estimates, as illustrated by the image below, from Pinero et al.

The amount of carbon stored in hydrates globally was in 1992 estimated to be 10,000 Gt (USGS), while a later source gives a figure of 63,400 Gt C for the Klauda & Sandler (2005) estimate of marine hydrates. A warming Gulf Stream is causing methane eruptions off the North American coast. Furthermore, methane appears to be erupting from hydrates on Antarctica, on the Qinghai-Tibetan Plateau and on Greenland. In just one part of the Arctic Ocean alone, the East Siberian Arctic Shelf (ESAS), up to 1700 Gt of methane is contained in sediments in the form of methane hydrates and free gas. A sudden release of just 3% of this amount could add over 50 Gt of methane to the atmosphere, i.e. some seven times what is in the atmosphere now, and experts consider such an amount to be ready for release at any time.

Importantly, methane levels have risen even more strongly than carbon dioxide levels. As the image at the top of this post shows, the current methane level is 250% its pre-industrial peak level, i.e. 1100 ppb above the pre-industrial peak level of 700 ppb. Historically, methane has only moved by some 300 ppb between a glacial and interglacial phase of the ice ages. IPCC/NOAA figures suggest that global mean methane levels have been rising by 5 or 6 ppb annually over recent years and there are some worrying indications that the rise of methane levels might accelerate even further.

To obtain mean methane abundance, measurements are typically taken at an altitude of 586 mb, as methane typically shows up most prominently at this altitude. Indeed, mean methane levels were highest at this altitude in April 2013, at just under 1800 ppb. Looking at mean global methane levels in April 2014 at this altitude, one could at first glance conclude that the situation had not changed much, and that 2014 methane levels had merely risen by a few ppb, in line with IPCC data. So, at first glance one might conclude that there may appear to be only a minimal rise (if any at all) in global mean methane levels when taking measurements at lower altitudes.

The image below illustrates this. What should be added is that the analysis used only selected altitudes and only used part of all data. So, further analysis may be necessary to verify these findings.

Importantly, closer examination of above graph shows that the situation is dramatically different when looking at the rise in methane levels at higher altitudes. A huge rise in mean methane levels appears to have taken place, to the extent that the highest mean level is now reached at 469 mb. Overall, the average rise in methane across the altitudes that are highlighted in the image is no less than 16 ppb.

The table below shows the altitude equivalents in mb (millibar) and feet.

56925 feet	44689 feet	36850 feet	30569 feet	25543 feet	19819 feet	14383 feet	8367 feet	1916 feet
74 mb	147 mb	218 mb	293 mb	367 mb	469 mb	586 mb	742 mb	945 mb

As the image below illustrates, this rise appears to go hand in hand with much higher peak readings, especially at higher altitudes. It appears that the additional methane originates from the higher latitudes of the Northern Hemisphere and has over the past few months moved closer to the equator, which is what typically occurs as methane rises in altitude.

Peak readings in above image are averages over April. On specific days, peak readings could be much higher, e.g. on April 28, 2014, methane levels were recorded as high as 2551 ppb at 469 mb.

As said, there appears to be a 16 ppb rise when comparing global mean methane levels between April 2013 and April 2014. Indeed, the culprit appears to be the rapid rise of methane emissions from hydrates that has been documented by this blog and that I estimated to amount to 99 Tg annually, as illustrated by the image below, from an earlier post.

So, it appears that the rise of methane in the atmosphere is accelerating. What can we expect? As temperatures can be expected to continue to rise and as feedbacks start to kick in, this may well constitute a non-linear trend. The image below shows a polynomial trend that is contained in IPCC AR5 data from 1955 to 2011, so they didn’t include this recent steep rise. Nonetheless, the polynomial trendline points at methane reaching mean global levels higher than 3000 ppb by the year 2030. If methane starts to erupt in large quantities from clathrates underneath the seafloor of the Arctic Ocean, this may well be where we are heading.

So, how high could temperatures rise? Worryingly, a non-linear trend is also contained in the temperature data that NASA has gathered over the years, as described in an earlier post. A polynomial trendline points at global temperature anomalies of 5°C by 2060. Even worse, a polynomial trend for the Arctic shows temperature anomalies of 4°C by 2020, 7°C by 2030 and 11°C by 2040, threatening to cause major feedbacks to kick in, including albedo changes and methane releases that will trigger runaway global warming that looks set to eventually catch up with accelerated warming in the Arctic and result in global temperature anomalies of 20°C+ by 2050.

Without action, it appears that the Antropocene will lead to extinction of the very human beings after which the era is named, with the Anthropocene only running from 1950 to 2050, a mere 100 years and much too short to constitute an era. In that case a better name would be the Sixth Extiction Event, as also illustrated by the image below, from an earlier post.

In conclusion, it’s high time that we start acting as genuinely wise modern human beings and commit to comprehensive and effective action as discussed at the Climate Plan blog.

Post by Sam Carana.