2. Glaciers melt, rains come, seas rise, people move.

 In the prior blog, we saw that the 20,000-year-old, latest global warming cycle called the Holocene saw global average temperatures rise steeply for the first 10,000 years, level off for 6,000, then begin a slow decline for the past 4,000 years. We also saw in that first graph that global warm-ups happen much faster (20-60,000 years) than global cool-downs (45-100,000 years). In addition, we saw in that graph that there were six inter-glacial warming peaks, three warmer than today’s (assumed) peak, and two peaks that were cooler than today’s (assumed) peak. We concluded expecting to see earlier civilizational, even monumental artifacts, from before Gobekli Tepe’s origins at 12,000 BP i.e. in the rapid warming years 20-12,000 BP following the last glacial maximum.

Now, let’s consider how much the temperature rose during those years 20-10,000 BP. Consider the figure below. Try to ignore the upper graph of CO2 concentrations tracking temperature over the period, and focus on the lower graph of average temperatures over the period ranging from 3°C to minus 9°C (37°F to 16°F). The average temperature in the Holocene has risen 10°C (18°F) so far, from 16°F to 34°F, while the mountains of ice (glaciers) were melting apace.

Temperature and CO2 over past 400,000 years
Temperature and CO2 over past 400,000 years

Graph Source

We mustn’t let our imaginations run wild in this scenario. But let’s do imagine the glaciers melting fast during that first 10,000 years after the glacial maximum, and slowly for the next 10,000 up to today. Where’s all that fresh water going to go? Some will evaporate into the air in the warmer temperatures, as the water carrying capacity of the air expands with the 10°C increase in Holocene temperature. Please understand that until the temperature rises to 0°C, water freezes and cannot be absorbed into air. So, as the temperature rises from -9°C to 0°C, no glacial water can be absorbed by the air. The Holocene average temperature never rises above 3°C (37°F)—but air is absorbing water above 0°C (32°F). Warm air rises and cools at a rate of 5.5°F per 1,000 feet (10°C per kilometer). As it rises and cools, humidity rises and finally reaches 100% where water is released upon further cooling. With the rising temperatures after reaching 0°C in the Holocene, the air is absorbing some percentage of the glacial melt water. It might seem small in the graph below, but there is an awful lot of air and the fact that the air is moving the moisture from one place to another allows rain to fall in previously dry places, facilitating the emergence of agriculture which can depend upon regular rain. 

air-moisture-holding-capacity-si
Air’s moisture-holding capacity rises as world warms

Graph Source

The water that is not absorbed into the air will run downhill, filling every low space it reaches, then overflowing until it reaches the sea. That melt water explains the emergence of large fresh water lakes and rivers where glaciers once covered the earth in the northern hemisphere, and their constant replenishment by rain until now. One way to get a feel for how much water was released from the glaciers and not sublimated or absorbed in the air is to look at the sea level changes since the last glacial maximum, and the Melt Water Pulses in raising the sea level (MWP in the graph below) Think about what could have happened during these huge pulses. The MWP-1C could have been the one that broke through from the Mediterranean to flood the valley that became the Black Sea. In September 2000, marine archaeologist Robert Ballard found a substantial wood shelter with neolithic tools down 95 meters on the bottom of the Black Sea.

Sea levels with 4 major pulses in Holocene
Sea levels rose with 4 major pulses as glaciers melted

Graph Source

One consequence of the sea level rise was to push the population living in the valley under what is now called the Persian Gulf to migrate ahead of the rising waters until they ended up in Mesopotamia. No marine archaeology has been done yet on the Persian Gulf, but there is a consensus that this population was forced out of the “valley” now under the Gulf.

Next time we’ll start reviewing data regarding Holocene flora: forestation and harvesting, plains and grain, precipitation and agriculture, and desertification.

2 Responses

  1. Marilyn J.
    Marilyn J. at |

    To a non-science person such as myself, this was extremely interesting – and I could follow it all the way through! I didn’t pay much attention to the graphs in Blog post 1, but these made sense. Now I have scientific explanations to balance my awe at the sight of Lake Louise in Banff National Park, or Mono Lake near Yosemite.

    Reply

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