The Cause of Ice Ages and Global Warming on Planet Earth
February 2018
SNIPPET:
The long collective trudge through the intricacies of field data and models gradually increased understanding of all the interacting forces that drive climate cycles. By around 2013 computer models had finally advanced to the point where modelers could get convincing glacial cycles. They ran a climate model to take account of variations in sunlight and the rise and fall of CO2, then took snapshots from this model and fed them into a model for ice-sheet behavior and fed the result back into their climate model. They found that the approximate 100,000-year cycle might be explained by the slow settling of rock under the colossal weight of the North American ice sheet.
After several 23,000-year cycles the Earth's crust sagged so far that the ice's surface was at a low enough altitude to melt in summer — but only when orbital conditions brought increased sunlight in northern latitudes.(57a) This was a good start, but it was only one of several ideas about how a huge ice sheet might become unstable.
More work would be needed before the entire pattern of interactions among bedrock, ice sheets, oceans, and climate could be well understood.
The invaluable fruit of a century of ice ages research was the
recognition of how complex and powerful the feedbacks could be. An intriguing clue came from some especially good Antarctic ice core records that timed precisely the changes in the levels of CO2 and methane. The levels apparently rose or fell a few centuries after a rise or fall in temperature. Later and even more precise work reversed this, finding that the CO2 rise came first. Still later work concluded that if there was a lag either way, it was too small to be seen amid the noise.
All this drove home that it scarcely mattered which came first, for the chief effect of the Milankovitch-cycle orbital changes was to initiate a powerful feedback loop. The close of an ice age began when a shift in sunlight caused a rise of temperature, at first almost negligible. Over the next few centuries that would
make gas levels rise (above all, as it turned out, through evaporation of CO2 from the vast Southern Ocean). The greenhouse effect would then drive the planet's temperature a bit higher still, which would drive a further rise in the gas levels... and so forth.(58)
=>CO2 greenhouseOur current situation was altogether different. The
warming was not started by a small shift of sunlight, as in previous epochs. Our
addition of gases to the atmosphere was initiating the process, with the
temperature rise 🌡️
lagging a few decades behind the rise of gas levels.
Emissions were surging far more swiftly than anything in the Pleistocene record.
Already by the 1980s the levels of greenhouse gases had climbed
far higher than anything seen for many millions of years. Even if we stopped our emissions, would feedbacks drive things higher on their own? There were disturbing signs that feedbacks were indeed kicking in. Drying forests and warmer seawater were
getting less efficient at taking CO2 out of the air, and
methane was seen bubbling up from Arctic wetlands.
<=CO2 greenhouseBy the start of the 21st century, it was clear that the connection between global temperature and greenhouse gas levels was a major geological force. All through the Pleistocene, the greenhouse gas feedback had turned the planet's orbital cycles from minor climate variations to grand transformations that affected all life on the planet. The geological record gave a striking verification, with wholly independent methods and data, of the processes that computer models were predicting would bring a
rapid and severe global warming — a disruption of climate exceeding anything seen since the emergence of the human species.Read more: The Discovery of Global Warming