Climate science

Guest post: The state of the Greenland ice sheet in 2015

  • 04 Sep 2015, 14:20
  • Dr Ruth Mottram & Dr Peter Langen

A guest post from climate scientists Dr Ruth Mottram and Dr Peter Langen from the Danish Meteorological Institute.

Dramatic decline in  Arctic sea ice has become an eye-catching barometer of our warming climate. But the Greenland ice sheet, which is often confused with sea ice, has exhibited no less dramatic - but less talked about - changes in the past few years.

Stretching out over 1.7m square kilometres, the Greenland ice sheet is the second largest land ice mass on the planet, after Antarctica. It's an area of solid ice and snow about the size of Libya.

The end of August heralds the completion of the summer melt season for another year and is an opportune time for an annual checkup. So is the Greenland ice sheet in rude health, or looking under the weather?

Give and take

An ice sheet is a vast, slow-moving mass of fresh water. It differs from a glacier or an ice cap because it isn't confined by the land it sits on. But, like all glaciers, it gains ice by the accumulation of snowfall in the winter and loses it from surface melting during summer and the 'calving' of icebergs that break off into the sea at its edges.

The melting and calving of ice is collectively called "ablation". If more snow accumulates than is ablated away, the ice sheet will grow and advance. Conversely, if more ice melts and calves than is replenished, it will retreat and shrink.

It's worth noting that melting at the surface doesn't necessarily mean that an ice sheet will lose mass. The meltwater percolates through to the lower layers of the sheet and often refreezes. But when snow and ice melts, it gives off heat, which warms up its surroundings making subsequent melting more likely.

Surface melting also affects how much of the Sun's energy the ice sheet reflects - known as the albedo effect. Bright white fresh snow reflects sunlight more efficiently than the much darker bare glacier ice underneath. So, if the snow melts, the ice sheet surface absorbs more energy and warms up more quickly.

Melt can occur at surprisingly high elevations on Greenland ice sheet. For example, observation stations regularly record melting at an altitude of 1,800m near Kangerlussuaq in western Greenland - even during cool summers.

In 2012, which was a record year for melting on Greenland, about 97% of the ice sheet was observed to be melting at some point - even at the summit at an altitude of 3,200m.

Greenland -ice -sheet -aerial -view

Aerial shot of eastern Greenland. Credit:  Shutterstock

Balancing act

Comparing the total snowfall and surface melt against each other gives you the "surface mass balance". At the Danish Meteorological Institute (DMI), we combine data from observation stations on Greenland with output from a numerical weather forecasting model to estimate the surface mass balance for the whole ice sheet for every single day.


New map reveals ‘astronomical’ scale of human impact on forests

  • 02 Sep 2015, 18:00
  • Robert McSweeney

New research suggests there are just over 3tn trees on Earth, which is eight times more than scientists previously thought. But this isn't the good news it sounds, as humans are cutting down over 15bn trees every year. On balance, once growth of new trees is taken into account, that means our forests are shrinking by around 10bn trees each year.

The sheer scale of deforestation means we're eating into the amount of carbon locked up in the world's forests, the study suggests.

Seeing the forest and the trees

Using satellite images, scientists have previously estimated that there are around 400bn trees on Earth.

But satellites can only 'see' trees from above. This means that scientists have to estimate tree numbers based on images of how much of the Earth's surface is obscured by the leaves and branches of the trees - known as the canopy.

The new study, published today in Nature , takes a different approach.

The researchers collected measurements on the ground from almost 430,000 forests in over 50 countries and combined this information with satellite images. Lead author, Dr Thomas Crowther from Yale University, explained at a press briefing yesterday:

"We started to collect information about specific plots where someone has physically been on the ground, counting the number of trees in an area. And then we can link that to the satellite information."

By matching up counts of tree numbers to the satellite images of the same forest plots, the researchers could then map similar types of forests for every square kilometer of the rest of the Earth's surface:

"If we know what ten trees looks like in a field [on a satellite] and what 100 trees looks like, we can start to then predict what the picture in a new area looks like."

Using this method, researchers estimate that there are actually 3.04tn trees on our planet today - or 422 per person. The video below from Nature describes the new research.


Credit: Nature video.


Warming tropical oceans could see ‘widespread and intense’ species loss, study warns

  • 31 Aug 2015, 16:00
  • Robert McSweeney

The tropics could see a huge drop in biodiversity as marine life heads for cooler waters, a new study suggests.

Rising sea temperatures could push fish, molluscs and crustaceans towards higher latitudes, the researchers find. But species that can't move fast enough are likely to face local extinction if emissions remain very high, the lead author tells Carbon Brief.

Sea surface temperature

There are around  230,000 known species swimming, floating and crawling around the world's oceans. A key factor in where they are located is the temperature of the water.

The map below shows the distribution of marine species around the world. You can see from the areas shaded yellow or red that there tends to be a larger number of species in warmer, tropical waters than in cooler waters towards the poles.

Total -richness -2006Current distribution of marine species in the world's oceans (as of 2006). Orange and red areas show areas where number of species is high, while blue areas show areas where biodiversity is low. Graph on right-hand side shows number of species by latitude - where the further to the right the line is, the more species found. Source: García Molinos, et al. (2015).

But warming oceans may see marine life venturing away from their current habitats. A study from earlier this year, for example, found that warming waters in the Arctic could allow more species to bridge the chilly divide between the Atlantic and Pacific oceans and mix more easily.

The new study, published in Nature Climate Change, looks at how rising sea surface temperatures could affect how species are spread across the world's oceans.

Researchers modelled the impact of future temperature change on around 13,000 marine species - over 12 times more than any other study.

Habitat range

The researchers first estimated what temperatures each species can tolerate - based on how cold and hot their existing habitats get and how far their existing ranges stretch.

The researchers then modelled how their habitat ranges could change as the oceans get warmer. Of the four pathways of future climate change developed for the Intergovernmental Panel on Climate Change (IPCC ), the study uses two: a moderate scenario where global emissions level off around the middle of the century ( RCP4.5) and the scenario with the highest emissions of the four ( RCP8.5). Global emissions are currently  tracking just above this scenario.