Tipping points - Lecture 6 (climate tipping points)

What is the west antartic ice sheet?

a marine based ice sheet

what does a marine based ice sheet mean?

it is vulnerable to melting from the warm ocean below

why are ice sheets melting

because it is much quicker to melt an ice sehet than it is to grow one - once ice sheets are destabilised it takes them thousands of years to re grow

what is the marine ice sheet instability feedback

The bed slopes into the continent. The rest is open ocean. Grounding line – where the ice sheets stops touching the bed. The ice sheet becomes an ice shelf. The ocean is gradually eroding under the ice shelf – pushes back the ice shelf. More water and more surface area to melt more. 

 

Melting from below makes this very vulnerable. 

what is marine ice cliff instability

Regardless of the slop of the bed. There is a fracturing of the surface of the ice shelf. The ice shelves are protecting the ive sheets (attached to continent). The ice shelves form little fractures from the ice which causes the ice shelf to break off and the grounding line retreats. Causes the pressures that cause the increase melting of the ice sheets. Another feedback mechanism that promotes the retreat of the ice sheet. 

 

Both are only affecting Antartic ice sheets e.g greenland is melting very differently to how antarctic is melting. Because greenland is sitting above sea level 

what do ice shelves do

they hold back ice sheets but they are destabilising

what does the IPCC project about the antartic contribution to global mean sea level

they will contribute around 7ca to the global mean sea level by2100

what are the major drivers to greenlands ice melt

• rapid increase of outlet glaciers due to basal sliding

  • moulins divert surface meltwater to the base of the glacier, lubricating the movement

why is artics sea ice thickness important

the thicker it is the more resistant to melt it is

how does surface albedo become a positive feedback

if there is higher temp = less ice = more ocean (darker) = becomes hotter as doesnt reflect as darker

what is happening to the AMOC

it is being threatened to collapse due to a warmer world.

what is happening to the west african monsoon system

it bring in rainfall to west africa and sahel - the dry season is nov -may and wet season june - sep

however,

what is the evidence of a greener sahara

• 10 million paintings and engravings have been found across the Sahara. Depicting pastoral scenes, elephants, giraffe, hippos, antelope etc.

• Dust blown from the Sahara as a proxy for vegetation and moisture in the Sahara. The abundance of windblown dust preserved in deep-sea sediments off the Northwest African coast can be used as an independent proxy of North African rainfall. More dust = more desert!

• Huge palaeo lakes covered large parts of North Africa – one example is megalake Chad area: >330,000km2 - larger than the size of Poland! The boundaries of this ancient lake have been recognized by beaches, spits and deltas

• Pollen extracted from lake sediments shows the region during early Holocene was covered in grassland and shrubs

what happened to the green sahara as the monsoon weakened

the green sahara dissapeared and peoplle migrated south or fell upon reliable water sources, particulary the nile. there have been 230 green sahara events over the last 8 million years

how does changing orbit afect the african mondoo

the northern hemisphere summer found itself closest to the Sun. Summer heat gradually increased across North Africa, peaking between 11,000-10,000 years ago. Insolation reached a peak of 8% higher than today.

Warmer land surface meant more air penetrating into the continent. 8% increase in summer insolation would result in a 50% increase in rainfall. But 50% more rain wasn’t enough to fill the large lakes

how are there vegetation - precipitation feedbacks

radiation feedbacks = more energy for convection

evaporation from vegetation and lake surfaces feed the water back into the atosphere

biosphere tipping points

• marine bio pump

• tropica coral reefs

• amazon rainforest

• boreal forest

Name the circulation pattern tipping point

• el ninp

• west african monsoon

• indian summer monsoon

• thermohaline circulation

• sw north african

name the cryosphere tipping points

• west antartic ice sheets

• east antartic basins

• methane clathrates

• artic sea ice

• greenland ice sheet

• yedoma permafrost