Antarctic climate change and the environment

TLDR: The Southern Hemisphere climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system as discussed by the authors.

Abstract: The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.

Figures

Fig. 1. a. Overview map of Antarctica indicating key regions or

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Convey, P. and Bindschadler, R. and di Prisco, G. and Fahrbach, E. and Gutt, J. and Hodgson, D. A. and
Mayewski, P. A. and Summerhayes, C. P. and Turner, J. and ACCE Consortium (incl Bentley M.J.), (2009)
'Antarctic climate change and the environment.', Antarctic science., 21 (6). pp. 541-563.
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http://dx.doi.org/10.1017/S0954102009990642
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Antarctic Science 21(6), 541–563 (2009) & Antarctic Science Ltd 2009 doi:10.1017/S0954102009990642
Review
Antarctic climate change and the environment
P. CONVEY
1
*, R. BINDSCHADLER
2
, G. DI PRISCO
3
, E. FAHRBACH
4
, J. GUTT
4
, D.A. HODGSON
1
,
P.A. MAYEWSKI
5
, C.P. SUMMERHAYES
6
, J. TURNER
1
and THE ACCE CONSORTIUM
7
1
British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 0ET, UK
2
Hydrospheric and Biospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
3
Institute of Protein Biochemistry, National Research Council, Via Pietro Castellino 111, I-80131 Naples, Italy
4
Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
5
Climate Change Institute, Sawyer Research Facility, University of Maine, Orono, ME 04469, USA
6
Scientific Committee on Antarctic Research, Scott Polar Research Institute, Lensfield Road, Cambridge CB2 1ER, UK
7
see Appendix 1
*pcon@bas.ac.uk
Abstract: The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual,
and is closely coupled to other parts of the global climate system. We review these variations from the
perspective of the geological and glaciological records and the recent historical period from which we have
instrumental data (, the last 50 years). We consider their consequences for the biosphere, and show how
the latest numerical models project changes into the future, taking into account human actions in the form
of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an
essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.
Received 20 July 2009, accepted 26 September 2009
Key words: Antarctica, biology, environmental change, geology, glaciology, Southern Ocean
Introduction
The central theme of the newly published ‘Antarctic Climate
Change and the Environment’ (ACCE) report (Turner et al.
2009a; http://www.scar.org/publications/occasionals/acce.html)
is to describe the way in which the physical climate system
of the Antarctic has varied on the geological timescale, how
it is changing during the instrumental period, and how that
variation affects or may affect life. In explicitly including
consideration of biological evidence and implications,
the ACCE report provides a logical advance from ‘The
State of the Antarctic and Southern Ocean Climate System’
(SASOCS) report (Mayewski et al. 2009), whose purpose
was to review ‘developments in our understanding of the
state of the Antarctic and Southern Ocean climate and its
relation to the global climate system over the last few
millennia’. For the purposes of the ACCE report and the
current review, the Antarctic region (Fig. 1) is considered to
include the continent of Antarctica, its offshore islands
including the sub-Antarctic islands, the surrounding Southern
Ocean including the Antarctic Circumpolar Current (the
northern boundary of which is the sub-Antarctic Front), and
Southern Ocean islands that lie north of the sub-Antarctic
Front and yet fall into SCAR’s area of interest, including Ile
Amsterdam, Ile St Paul and Gough Island.
Antarctica is renowned as being the highest, driest, windiest
and coldest continent, boasting the lowest recorded temperature
on Earth, -89.28C, at Russia’s Vostok Station on the Polar
Plateau (Turner et al. in press). The continent with its ice
shelves and islands covers an area of approximately
14 3 10
6
km
2
, which is about 10% of the land surface of the
Earth. Most of the continent, apart from the northern part of the
Antarctic Peninsula lies, south of the Antarctic Circle (at
latitude 66833'39''S), beyond which there is 24 hours of
continuous daylight at the summer solstice in December,
and 24 hours continuous darkness at the winter solstice
in June.
Moving inland, the surface rises rapidly and the
continent has the highest mean elevation of any continent
on Earth, at around 2200 m. It is dominated by the
Antarctic Ice Sheet, a contiguous mass of glacial ice that
rests on the continent and surrounding seas, contains
around 30 x 10
6
km
3
of ice or 70% of the Earth’s freshwater
and covers over 99.6% of the continent. The ice sheet is
made up of three distinct glaciological zones, the East
Antarctic (EAIS, covering an area of 10.35 x 10
6
km
2
),
West Antarctic (WAIS, 1.97 x 10
6
km
2
) and Antarctic
Peninsula (APIS, 0.52 x 10
6
km
2
) ice sheets. The EAIS
includes the high Polar Plateau, while the WAIS is lower
in altitude. The EAIS and WAIS are separated by the
Transantarctic Mountains, which rise above the surrounding
ice sheet to a maximum height of 4528 m, and extend from
Victoria Land to the Ronne Ice Shelf. The Antarctic Peninsula
is the only part of the continent that extends a significant
way northwards from the main ice sheet. It is a narrow
mountainous region with an average width of 70 km and a
541

mean height of 1500 m. The northern tip of the Peninsula is
close to 638S, forming a barrier that has a major influence
on the oceanic and atmospheric circulations of the high
southern latitudes.
The ice sheet is nourished at its surface by deposition of
snow and frost which, because of the year-round cold, does
not melt but accumulates year-on-year. As the surface snow is
buried by new snowfall, it is compressed and eventually
transformed into solid ice, a process that captures a chemical
record of past climates and environments. In places, the
deepest ice may be more than one million years old. The ice
gradually flows down to the edge of the continent in a number
of ice streams and outlet glaciers that move at speeds of up to
a few kilometres per year, transporting around 2000 x 10
9
tonnes of ice per year from the interior to the coast. Once the
ice streams reach the edge of the continent they either calve
into icebergs, which drift away within the surrounding seas, or
start to float on the ocean as ice shelves, which can be several
hundreds of metres thick. The ice shelves constitute 11% of
the total area of the Antarctic, with the two largest being the
Ronne–Filchner Ice Shelf in theWeddellSeaandtheRossIce
Shelf in the Ross Sea, which have areas of 0.53 x 10
6
km
2
and
0.54 x 10
6
km
2
respectively.
The formation of deep and bottom ocean waters of
Antarctic origin occurs over the continental shelves of the
continent. The process starts with the formation of sea ice
over the continental shelf alon g the front of the ice shelves,
especially where winds move newly formed sea ice
seawards away from the ice shel f to form polynyas. Sea
ice formation rejects sal t, leading to the formation of High
Salinity Shelf Water (HSSW). This can mix with Warm
Deep Water (WDW) from the mid-levels of the sub-polar
gyres, either over the shelf in areas where WDW penetrates
onto the shelf in modified form, or over the slop es in
regions where the dense water can spill off the shelf more
directly (e.g. Gill 1973, Foster & Carmack 1976, Gordon
1998). These processes lead to the formation of deep and
bottom waters, often termed collectively as Antarctic
Bottom Water (AABW); these spread northwards to cool
and aerate most of the global deep ocean floor, and provide
a fairly stable thermal environment for bottom dwelling
(benthic) organisms. A further route for AABW formation
exists, whereby dense shelf waters penetrate beneath the
floating ice shelves. This water mixes with fresh meltwater
from the base of the ice shelves to form slightly less dense
Ice Shelf Water (ISW) (Nicholls et al. 2009). When this
ISW cools it becomes denser and exits from beneath the ice
shelves, flowing down the continental slope and eventually
contributing to the AABW layers.
In the seas surrounding Antarctica, the continental shelf
is unusually deep, reaching 800 m in places, as a side effect
of the continent being depressed by the weight of the ice
sheets. The shallower parts are impacted by modern
icebergs, most intensely at depths up to tens, but
occasionally to several hundreds, of metres. More than
Fig. 1. a. Overview map of Antarctica indicating key regions or
locations within the continent referred to in the text.
Abbreviations: EM 5 Ellsworth Mountains, MS 5 McMurdo
Sound, PIG 5 Pine Island Glacier. b. Schematic map of
major ocean currents south of 208S(F5 Front, C 5 Current,
G 5 Gyre), showing: i) the Polar Front and sub-Antarctic
Front, which are the major fronts of the Antarctic
Circumpolar Current, ii) Other regional currents, iii) the
Weddell and Ross Sea gyres, and iv) depths shallower
than 3500 m shaded. In orange are shown a) the cyclonic
circulation west of the Kerguelen Plateau, b) the
Australian-Antarctic Gyre (south of Australia), c) the slope
current, and d) the cyclonic circulation in the Bellingshausen
Sea, as suggested by recent modelling studies and
observations.
542 PETER CONVEY et al.

95% of the shelf lies at depths beyond the reach of the
scouring effects of sea ice or wave action, and is also below
the reach of sunlight. The seabed features form key elements
of the habitats of marine organisms, and constrain ocean
circulation.
In a visible contrast with the simple ecosystems and
expanses of apparently barren ground that characterize the
physically isolated ecosyste ms on land , many benthic
organisms live on the Antarctic continental shelf, which
comprises almost 15% of the global continental shelf area -
in total around 4.6 3 10
6
km
2
. Biomass and diversity in
these marine ecosystems may be second only to those of
tropical coral reefs. Floating ice shelves cover about one
third of the shelf, while the rest is covered by sea ice for
around half the year. Both the sea and the seabed below the
ice shelves remain among the least known habitats on
Earth, owing to their inaccessibility.
The continent is surrounded by the sea ice zone, where,
by late winter, the ice on average covers an area of
20 3 10
6
km
2
, which is more than the area of the continent
itself. At this time of year the northern edge of the sea ice is
close to 608S around most of the continent, and near 558Sto
the north of the Weddell Sea. Unlike the Arctic, most of the
Antarctic sea ice melts during the sum mer so that by
autumn it covers only an area of about 3 3 10
6
km
2
. Most
Antarctic sea ice is therefore first-year ice up to 1–2 m
thick, with the largest area of thicker multi-year ice being
over the western Weddell Sea.
The Antarctic plays a central role in the global climate
system. This is driven by solar radiation, most of which
arrives at low latitudes, with the Equator receiving about
five times as much radiation annually as the poles and so
creating a large Equator-to-pole temperature difference.
The atmospheric and oceanic circulations respond to this
large horizontal temperature gradient by transporting heat
polewards. The planet’s climate system can be regarded as
a thermodynamic engine, with the low latitude areas being
the heat source and the polar regions the heat sink.
The wind system and the meridional gradient in buoyancy
input give rise to the Southern Ocean’s Antarctic Circumpolar
Current (ACC), which links the major ocean basins in the
global ocean system (Fig. 1b). Under the influence of the
Coriolis force of the Earth’s rotation, westerly winds cause
Southern Ocean surface waters to be diverted northward. The
surface waters are replaced by Circum-polar Deep Water
upwelling from below, derived ultimately from North Atlantic
Deep Water. The northward moving surface water in the ACC
then sinks to produce Antarctic Intermediate Water and sub-
Antarctic Mode Water. A separate component of the upwelled
water spreads southward and into the sub-polar gyres, where it
is involved in AABW formation. These branches are,
respectively, the upper and lower limbs of the overturning
circulation in the Southern Ocean, and lead to the seas around
Antarctica being especially important in the global ocean
overturning (Rintoul et al. 2001).
The marine carbon cycle can be described in terms of
anthropogenic and natural carbon cycles, but from an oceanic
perspective these two are treated almost exactly the same. The
anthropogenic carbon cycle includes the emissions of CO
2
into the atmosphere that have continued at an increasing rate
since the start of the industrial revolution. The natural carbon
cycle includes the behaviour of the carbon in the ocean prior
to the industrial revolution. It is estimated that 30% of the
total anthropogenic emissions annually are taken up and
sequesteredbytheocean(Sabineet al. 2004). The Southern
Ocean plays a key role in the global carbon cycle. The
upwelling deep water south of the Polar Front brings to the
surface dissolved nutrients and carbon dioxide (CO
2
), and
releases this gas to the atmosphere. In contrast, water masses
sinking north of the Polar Front take up CO
2
from the
atmosphere, including some of the CO
2
released to the
atmosphere by human activities. These complementary
processes make the Southern Ocean both a source and a
sink for atmospheric CO
2
, with any change in this balance
being of global significance.
Because of its upwelling nutrients, the Southern Ocean is
the world’s most biologically productive ocean, although
its productivity is also limited by the low availability of
micronutrients such as iron, except around the islands that
are scattered through the ACC. As a result the Southern
Ocean is classified as a High Nutrient Low Chlorophyll
(HNLC) region. Through photosynthesis, the growth of
phytoplankton extracts CO
2
from the atmosphere and
pumps it to the seabed or into subsurface waters through
the sinking of decaying organic matter. Without this
process, and without the solution of CO
2
in cold dense
sinking water near the coast, the build up of this gas in the
atmosphere would be much faster.
In terms of understanding the biology of the Earth System,
the poles fulfil a very special role. Their slowly changing
physico-chemical features have engineered life processes so
that organisms surviving the ensuing severe selection pressures
can prosper in such extreme habitats. It is unreasonable to
investigate life in Earth’s extreme environments without
also addressing the impacts of current climate changes on
organisms whose adaptations to climatic conditions have
slowly evolved over geological time to reach equilibrium. This
equilibrium is delicate, and the Antarctic and its biota currently
command increasing attention in a world attuned to
changes in global climate, loss of biological dive rsity and
depletion of marine fisheries. The links between long-
and short-term global climate change and evolution are
among the least understood natural events in the history of
the Earth. Excellent examples are available for study in
the Antarctic. Understanding the impact of past, current and
predicted environmental change on biodiversity and the
consequences for Antarctic ecosystem adaptation and
function must be a primary goal of research today.
The critical examination of Antarctic ecosystems
undergoing change provides a major contribution to the
ANTARCTIC CLIMATE CHANGE AND THE ENVIRONMENT 543

understanding of evolutionary processes of relevance to
life on Earth. How well are Antarctic organisms able to
cope with daily, seasonal and longer-term environmental
changes? Will climate change result in relaxation of
selection pressure on genomes, or tighter constraints and
ultimately extinction of species and populations? The
Antarctic holds great potential for studies in evolutionary
biology, playing an important role in understanding the
biological response to climate change within the whole
Earth system. There is evidence that climate change, and
modifications of the Earth system, occur in the polar
regions at faster rates than elsewhere. The uniquely adapted
fauna and flora of these regions are vulnerable to shifts in
climate. Therefore, it is urgent to establish the state of
Antarctic ecosystems, and in particular their diversity. To
assess reliably the extent of future changes in polar
ecosystems, integration of studies and data is required
across continental scales to bring undisputable evidence
of change in ecosystem structure, functioning or services.
Unprecedented international collabo rative research effort in
the recent International Polar Year framework and beyond
will provide scientists, environmental managers and
decision-makers with a solid benchmark against which
future changes can reliably be assessed.
With this brief background illustrating both the central role
of the Antarctic in the global climate system and oceanic
processes, and that of its biology in understanding the
potential responses of biota and ecosystems to change
processes, the publication of the ACCE report (Turner et al.
2009a) is a pivotal and timely event, providing an essential
Southern Hemisphere companion to the Arctic Climate Impact
Assessment (ACIA; Arctic Council 2005). The purpose of this
present review is to provide an accessible overview of the
major elements and conclusions of the ACCE report. The
report is extensively referenced and, while we provide salient
literature sources relevant to each subject discussed here, we
refer the reader to the report itself for more thorough access
to literature across the many disciplines covered. The online
version of the report (http://www.scar.org/publications/
occasionals/acce.html) is also intended as a living document,
which will be updated over time.
The geological dimension (deep time)
Studying the history of Antarctica’s climate and environment
provides the context for understanding present day climate
and environmental changes. It allows researchers to determine
the processes that led to the development of our present
interglacial period and to define the ranges of natural climate
and environmental variability on timescales from decades to
millennia that have prevailed over the past millions of years.
Knowing the boundaries of this natural variability enables us
to identify when present day changes exceed the natural state.
Concentrations of the greenhouse gas CO
2
in the
atmosphere ranged from roughly 3000 ppm (parts per
million) in the Early Cretaceous 130 million years ago
(Ma) to about 1000 ppm in the Late Cretaceous (at 70 Ma)
and Early Cenozoic (at 45 Ma), leading to global
temperatures 68 or 78C warmer than present (e.g. Royer
2006). These high CO
2
levels were products of the Earth’s
biogeochemical cycles. During these times there was little
or no ice on land. The first continental-scale ice sheets
formed on Antarctica around 34 Ma, most probably in
response to a decline in atmospheric CO
2
levels caused by a
combination of reduced CO
2
out-gassing from mid-ocean
ridges and volcanoes, and increased carbon burial (Pearson
& Palmer 2000). This decline resu lted in a fall in global
temperatures to around 48C higher than today (DeConto &
Pollard 2003, Pagani et al . 2005). At maxima these early
ice sheets reached the edge of the Antarctic continent, but
were most probably warm er and thinner than those present
today. Further rapid cooling took place at around 14 Ma,
probably accelerated by the growing physical and thermal
isolation of Antarctica as other continents drifted away
from it, and as the Antarctic Circump olar Current (ACC)
developed (Flower & Kennett 1994). At that time the ice
sheet thickened to mor e or less its modern configuration.
During the Pliocene (5–3 Ma), mean global temperatures
were 2–38C above pre-industrial values, CO
2
values may
have reached 400 ppm, and sea levels were 15–25 m above
today’s (Jansen et al. 2007).
The earliest cold-climate marine fauna is thought to date
from the latest Eocene–Oligo cene (,35 Ma). The
establishment of the Polar Front, separating warm water in
the north from cold water in the south, created a barrier for
migration of shallow and open-water marine organisms
between the Antarctic and lower latitudes (Barnes et al.
2006). This promoted adaptive evolution to cold temperature
and extreme seasonality to develop in isolation, and led to the
current Antarctic marine biota, which is second only to coral
reefs in terms of species diversity and biomass (Clarke &
Johnston 2003). In contrast to marine faunas elsewhere, the
Antarctic fish fauna is dominated by a single, highly endemic,
taxonomic group - the notothenioid fish (Notothenioidei). The
evolution of antifreeze proteins and the loss of the oxygen-
carrying pigments haemoglobin and myoglobin in members
of a family in this suborder is a particularly advanced
adaptation to the environment (DeVries & Cheng 2005,
Eastman 2000, Sidell & O’Brien 2006). This dominance by
a single taxonomic group of fish provides a simplified natural
laboratory for exploring their adaptive evolution. Amongst
other groups typical of faunas of lower latitudes, crabs,
lobsters and sharks are largely absent. The development of sea
ice made the success of krill (which relies on it as a ‘nursery
ground’) possible and, consequently, shaped the higher
trophic (feeding) levels of the Southern Ocean ecosystem.
Deeper water faunas (Gutt 2007) are not subject to the same
degree of isolation provided by the ACC in the upper layers
of the ocean profile, and invertebrates inhabiting the deep sea
have been able to continue considerable exchange with
544
PETER CONVEY et al.

Frequently asked questions

Q1. What are the contributions in "Antarctic climate change and the environment" ?

The authors review these variations from the perspective of the geological and glaciological records and the recent historical period from which they have instrumental data (, the last 50 years ). The authors consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, the authors provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment. 

Q2. What have the authors stated for future works in "Antarctic climate change and the environment" ?

The availability of these data for research has been enhanced through a variety of data sharing agreements, further increasing their value, and the research community has become increasingly reliant on their existence and availability. However, the continuation of these exceptional resources can not be taken for granted and many of the most valuable sensors are already beyond their design lifetime, with replacement missions either not planned or planned for launches so far in the future that prolonged gaps in coverage are probable. These challenges should provide the basis for further development of spatially explicit numerical simulations of the state-of-the-art of the Antarctic ecosystem, and extrapolations from this - with the support of, or in combination with results from physical models - into the future, using various climate scenarios. The prospect of, for instance, slowly going blind to ice sheets at precisely the moment when their behaviour has suddenly become very dramatic carries with it the undesirable consequences that, not only will the authors not be able to follow the continuing evolution of areas already changing dramatically, but they will not be able to detect new areas of change at an early stage, limiting their ability to understand the causes of these changes. 

Q3. What causes the Southern Ocean surface waters to be diverted northward?

Under the influence of the Coriolis force of the Earth’s rotation, westerly winds cause Southern Ocean surface waters to be diverted northward. 

Q4. What is the key challenge to the research community?

A key contemporary challenge to the research community is to incorporate physiological/ biochemical approaches into the field of evolutionary biology and ecology, taking advantage of the power of genomic and other ‘omic’ technologies. 

Q5. What is the magnitude of the uptake of CO2 by the ocean?

The magnitude of the uptake of CO2 from the air by the ocean will depend on how the ocean responds to increases in ocean warming and stratification, which can drive both increases in CO2 uptake through biological and export changes, and decreases through solubility and density changes. 

Q6. What are the main components of the projections?

The projections rely on continued data streams from satellites and field observations, which are crucial to quantifying the rapid rates of cryospheric change. 

Q7. Why is the ventilation of the ocean enhanced?

Ocean ventilation could be enhanced because of the surface divergence induced by the increase in wind stress projected during the 21st century. 

Q8. What is the purpose of the bridges between different disciplines and international programmes?

Bridges between different disciplines and international programmes will provide a legacy of knowledge for future generations in the form of a comprehensive information system. 

Q9. What is the key trait of many marine animals?

Being typically ‘stenothermal’ (able only to live within a limited range of temperature, contrasting with ‘eurythermal’ organisms that can maintain function across a wide range of temperatures) is a key trait of many Antarctic marine animals (Peck et al. 2006). 

Q10. How many tonnes of biomass were killed in the early 20th century?

As an example, about 300 000 blue whales were killed within the span of a few decades in the early- to mid-twentieth century, equivalent to more than 30 million tonnes of biomass. 

Q11. What is the main mode of variability in the atmospheric circulation of the Southern Hemisphere?

The major mode of variability in the atmospheric circulation of the high southern latitudes is the Southern Hemisphere Annular Mode (SAM), a circumpolar pattern of atmospheric mass displacement (that can be measured by barometers) in which intensity and location of the gradient of air pressure between mid-latitudes (high pressure) and the Antarctic coast (low pressure) changes in a non-periodic way over a wide range of timescales (Marshall 2003). 

Q12. How much warming is projected to occur in the troposphere?

The annual mean warming rate in the troposphere at 5 km above sea level is projected to be 0.288C per decade (T. Bracegirdle, personal communication 2009), somewhat less than the forecast surface warming. 

Q13. What is the effect of prey availability on whale demography?

Prey availability, itself correlated with changes in ocean climate (Trathan et al. 2006), may also have direct effects on whale demography, as has been demonstrated for southern right whales (Leaper et al. 2006). 

Q14. How much is the temperature of the Antarctic water predicted to warm during the year?

The bottom waters along the continental margin to a depth of 4000 m are predicted to warm during the whole year by around 0.258C. 

Q15. What are the reasons for the inaccuracies in the ACC transport?

These clear inaccuracies have been attributed to models incorporating too low a zonal wind stress, locating the maximum winds in the Southern Ocean too far north, or errors in the simulation of the ocean density gradient, partly due to problems in estimating the export of North Atlantic Deep Water (NADW) (Russell et al. 2006a). 

Q16. How many models are within 20 Sv of the ACC transport?

Only two models among the nineteen analysed by Russell et al. (2006a) generate transport values for the ACC that are within 20% of those estimated from observation (135 Sv), although most of the simulated values are within 50 Sv of this estimate.