Two years ago this month I posted Global temperature, the North Atlantic cool patch and the Gulf Stream. The cool patch was still there, lasting throughout the 2018 northern summer:
This now needs to be recognised as an enduring feature associated with the slowdown of the overturning ocean circulation AMOC (Atlantic Meridional Overturning Circulation) which James Hansen and Makiko Sato say is having an effect on the east coast hurricanes which have been so prominent during this summer.
The cool patch is thought to be due to a combination of sea ice export from the Arctic, the Greenland melt, and increased precipitation and river runoff.
Normally the Gulf Stream current becomes increasingly salty as the prevailing winds evaporate the sea water travelling across the North Atlantic. Near the north-east the heavier saltwater sinks, driving the thermohaline current:
Now the just-melted freshwater is diluting the salty water, inhibiting the sinking. Indeed the whole circulation system is slowing down. The system of currents bringing warm water to the North Atlantic has been in operation at least since the closing of the Panama Isthmus 2.8 million years ago. This closure has been called the most important geological event in the past 60 million years, because it makes northwestern Europe as much as 10 degrees C warmer than it otherwise would be.
How this may play out over the next century was addressed in 2016 in a 52-page paper by James Hansen and others – Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2°C global warming could be dangerous. The paper is complex, and impossible to do justice in a blog piece, but I had a go in Hansen worries that all hell will break loose. The focus there was on the possibility that giant storms could occur in line with evidence of what may have happened during the last interglacial, the Eemian, roughly 115 to 130 thousand years ago.
Hansen and Sato now say that the slowdown of AMOC is currently contributing to storminess in three ways.
First, sea level rise (SLR) is enhanced along the north American coast by the AMOC slowdown:
The slowdown reduces the west-to-east upward slope of the ocean surface across the Gulf Stream, causing piling up of water on the East Coast. The combined sea level rise from these effects, which is also responsible for “sunny day flooding” on the Eastern Seaboard, makes hurricane storm surges greater.
Combining their second and third effects, there is a warm band south of the cool patch, where the warmer ocean makes for larger storms in physical size and slower tracking, which then delivers huge amounts of precipitation.
All this makes for a greater impact on humans, their environment and the structures they have built.
Tamino at Open Mind, who has been talking to hurricane experts, identifies the rapid intensification of hurricanes as a key factor.
Hansen and Sato say that the tracking patterns may be different, but that has not yet been studied and it is probably too early to say. They do say that the numbers of hurricanes making landfall in the US may not have increased, and looks to be down in the current decade:
Please note, the number hurricanes in the North Atlantic basin is an entirely different number.
At this point I don’t want to repeat the ground I covered in Hansen worries that all hell will break loose. Hansen et al have used direct observations, paleoclimate information and a climate model incorporating ice sheet decay in both hemispheres. They are aware that there is insufficient information to be certain about anything, but warn that during the Eemian there was SLR of 2-3 metres within several decades at one point, that ice sheet melting may on average double every 10, 20 or 40 years, (they think closer to 10) and that if AMOC shuts down it could take centuries to restart.
The IPCC Special Report on 1.5°C has SLR at 26 to 77 cm by 2100 for 1.5°C global warming, 10 cm more for 2°C. The Hansen paper contemplates the possibility of 1 metre by 2060, whereupon the surface temperature map might look like this:
If ice sheet melt keeps doubling we could have five metres of SLR by 2096, whereupon with all that cold water spread around surface temperature may look like this:
The cooler surface in those circumstances does not stop ice sheet decay, because warmer water still has access to ice shelves. The paper suggests that:
- global surface air temperature, although an important diagnostic, is a flawed metric of planetary “health”, because faster ice melt has a cooling effect for a substantial period.
They suggest Earth’s energy imbalance as a more fundamental climate diagnostic. The control knob for energy imbalance is CO2 and other GHGs which should be reduced to 350ppm as soon as possible.
Their warning is not about linear gradients of temperature, rather an earth system which could become quite chaotic to human perception and may become beyond control.
We should recognise that we have a tiger by the tail.
Earlier post – Trouble at the top of the world.