Hydrology, climate change and the cryosphere
Lecture 1: Climate change and the cryosphere
Cryosphere comes from the Greek word kryos, meaning cold, frost, or ice and sphaira, meaning
globe or ball.
- Snow water equivalent: how much water is stored in the snow. Properties of snow are
grain size, albedo, depth etc.
- Sea ice is also important, the extent, concentration, and thickness.
- The ice sheets play a very large role in sea level rise and climate change.
- Mountain glaciers
- Permafrost: average temperature is below zero for two years.
Changes in the cryosphere: dramatic change in mountain glaciers, but also melting of ice
sheets, there is sea level rise (0.8 mm/year observed), sea ice declining (4%/decade) and
thinning (2-3 m from 1980-2008), ice shelves instability (come from the ice sheet and float on
water and can contribute quite significantly to SLR), permafrost changes (area and volume) and
lake and river ice decreasing.
There is a strong coupling between
the ocean and the cryosphere. Ocean
heat and CO2 are coupled (about 90%
increase in heat is absorbed by the
ocean), which acidifies the oceans
which reduces the oxygen. A lot of
feedback and couplings.
Human and natural processes driving change
in the polar regions: whole system with
connecting components. Examples: resource
extraction, local people, freshwater systems,
ocean circulation, sea ice, marine
ecosystems, ice sheets and glaciers,
atmospheric feedbacks, commercial activity,
international cooperation, terrestrial
ecosystem and snow and frozen ground.
The Artic is warming much faster than Antarctica. Already 0.5 degree in 35 years. In Antarctica
still some cooling.
,With projected climate change, the warming in the polar regions is higher than in the average
around the world = polar amplification. The sea heats less quickly than the land.
Glacier mass balances: balance of zero is neutral and in a healthy condition (snowfall = melt). If
the mass balance is negative, it loses more mass than it gains per year (snowfall < melt). In
kg/m2/year. In Artic and Greenland it is negative. Antarctic is less clear, but also a bit below the
zero line.
In Greenland there is the most mass
change over the years, which
contributes a lot to sea level. Surface
mass balance (=melting) and dynamic
thinning (= ice flows down to the ocean
from ice cap) are important processes.
In Antarctica the processes are less
clear and it is very dynamic. In East
Antarctica there is even mass gain.
There are many different components, which all need to be taken into account in a systematic
way to get the whole view. For example, the snow cover extent anomaly, observed permafrost
temperature anomaly, discharge, runoff, projected snow cover and permafrost. The net result is
a sum of all these components.
Permafrost is difficult to model because it has to do with the energy balance on the surface,
which is difficult to measure. Permafrost change is expected to be from 25 to 75% volume
change between different climate models.
Trends in snow cover and mass are quite similar and in summer not much is happening because
there is no snow. For the snow cover November and December. For mass January to May. Snow
cover models capture quite well the seasonality (sometimes bit overestimated). There are
ranges in climate scenarios from around 5-60% in snow cover decline.
,Conclusions:
- The Greenland and Antarctic ice sheets are losing mass, accelerating global sea level
rise. They will continue to melt, committing the planet to long-term global sea level rise.
- Arctic sea ice is declining in every month of the year and is getting thinner.
- At global warming of 1.5°C, the Arctic Ocean will rarely be free of sea ice in September.
At 2°C warming, this will occur up to one year in three.
- Permafrost is thawing, with the potential of adding more greenhouse gases to the
atmosphere.
- With global warming limited to well below 2°C, around one quarter of near-surface
permafrost will thaw by 2100. If emissions continue to increase strongly, around 70%
near-surface permafrost could be lost.
- People living in the Arctic, especially indigenous peoples, are already adjusting their
travel and hunting activities to the seasonality and safety of land, ice and snow
conditions. Their success in adapting depends on funding, capacities and institutional
support
Sea level rise
Many processes contribute to sea level rise. If all the ice in Antarctica would melt, there would
be 58 meters of sea level rise, for Greenland this is 7 meters and for glaciers it is 0.5m. If you
look historically the glaciers are the largest contributor, but looking at the potential Antarctica is
the biggest driver. The ice sheets happen slowly, while glaciers melt very fast. If you have a lot of
melt on the ice sheet, the mass on these ice sheets decreases, so the mass pulls less on the
ocean, so the sea level rise happens the further away from the ice sheet.
When we look historically at the different drivers of sea level rise, the expansion of the ocean
water is a very large driver (when the water is warmer, the density is lower so the water expands).
Glaciers historically have contributed most because they respond rapidly. Then there is still
Greenland and antarctica contributions and land water storage. There is quite a bit of variation in
the projections especially in the extreme scenarios. For the mild scenarios, it is a kind of linear
change and in the extreme scenarios, it is more exponential.
, The ice sheets have always grown and shrunk. Hundreds of meters of sea level rise have
happened in the past. In the Holocene there is a very rapid ice sheet retreat.
- Melting driven by ocean temperature: when bedrock dips seaward or is flat, the retreat
stops when warming stops. When ice sheets retreats, less ice is released into the ocean.
When bedrock dips land inward, the retreat is quick and self-sustained. When ice sheet
retreats, more ice is released into the ocean and the ice sheet retreats further.
- Melting driven by air temperature: the ice sheet is very thick therefore its surface is very
high and the air at high altitude is very cold. As the ice sheets melts, its surface goes
down until it reaches a threshold, where the surrounding air is warmer and melts the ice
even more quickly.
Thwaites glacier: only this glacier can already have an effect of 60 cm of sea level rise. Large part
of the glacier is floating, which causes it to be sped up tremendously because the ground line is
retreating. The warm circumpolar deep warming can also get underneath, which accelerates the
process.
There is ice sheet instability, which would make very rapid sea level rise. There is a low
probability, but this can happen.
Extreme sea level events: due to projected global mean sea level (GMSL) rise, local sea levels
that historically occurred once per century (historical centennial events: HCE) are projected to
become at least annual events at most locations during the 21st century. The height of a HCE
varies widely, and depending on the level of exposure can already cause severe impacts.
Conclusions
- During the 20th century, the global mean sea level rose by about 15cm. Sea level is
currently rising more than twice as fast and will further accelerate reaching up to 1.10m
in 2100 if emissions are not sharply reduced.
- Extreme sea level events which now occur rarely during high tides and intense storms
will become more common.
- Many low-lying coastal cities and small islands will be exposed to risks of flooding and
land loss annually by 2050, especially without strong adaptation.
- Various adaptation approaches are already being implemented, including:: protection,
accommodation, ecosystem-based adaptation, coastal advance & managed relocation
- People with the highest exposure and vulnerability are often those with the lowest
capacity to respond.
