Extreme GHG and temperature rise rates question linear climate projections

Andrew Gilksen, September 7 2019

“We will be in a position where we set off an irreversible chain reaction beyond human control that will most likely lead to the end of our civilization as we know it”…

“Now we probably don’t even have a future anymore, because that future was sold so that a small number of people could make unimaginable amounts of money. It was stolen from us every time you said that the sky was the limit, and that you only live once. You lied to us. You gave us false hope. You told us that the future was something to look forward to. And the saddest thing is that most children are not even aware of the fate that awaits us. We will not understand it until it’s too late. And yet we are the lucky ones. Those who will be affected the hardest are already suffering the consequences. But their voices are not heard.”

Greta Thunberg

F. Summary and conclusions

  1. The current growth rate of atmospheric greenhouse gas is the fastest recorded for the last 55 million years.
  2. By the mid-21st century, at the current COrise rates of 2 to 3 ppm/year, a CO-e level of >750 ppm is likely to transcend the climate tipping points indicated by Lenton et al. 2008 and Schellnhuber 2009.
  3. The current extreme rise rates of GHG (2.86 ppm CO2/year) and temperature (0.15-0.20°C per decade since 1975) raise doubt with regard to linear future climate projections.
  4. Global greenhouse gases have reached a level exceeding the stability threshold of the Greenland and Antarctic ice sheets, which are melting at an accelerated rate.
  5. Allowing for the transient albedo-enhancing effects of sulphur dioxide and other aerosols, mean global temperature has reached approximately 2.0 degrees Celsius above pre-industrial temperatures.
  6. Due to hysteresis the large ice sheets would outlast their melting temperatures.
  7. Land areas would be markedly reduced due to a rise to Miocene-like sea levels of approximately 40±15 meters above pre-industrial levels.
  8. Cold ice melt water flowing from the ice sheets into the oceans at an accelerated rate is reducing temperatures in large tracts in the North Atlantic and circum-Antarctic.
  9. Strong temperature contrasts between cold polar-derived and warm tropical air and water masses are likely to result in extreme weather events, retarding habitats and agriculture over coastal regions and other parts of the world.
  10. In the wake of partial melting of the large ice sheets, the Earth climate zones would continue to shift polar-ward, expanding tropical to super-tropical regions such as existed in the Miocene (5.3-23 million years ago) and reducing temperate climate zones and polar ice sheets.
  11. Current greenhouse gas forcing and global mean temperature are approaching Miocene Optimum-like composition, bar the hysteresis effects of reduced ice sheets (Figure 4A).
  12. The effect of high atmospheric greenhouse gas levels would be for the next ice age to be delayed on a scale of tens of thousands of years, during which chaotic tropical to hyperthermal conditions would persist until solar radiation and atmospheric COsubsided below ~300 ppm.
  13. Humans will survive in relatively favorable parts of Earth, such as sub-polar regions and sheltered mountain valleys, where gathering of flora and hunting of remaining fauna may be possible.
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