Analysis

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

  • 02 Sep 2015, 18:00
  • Robert McSweeney
Environmental damage to rainforest in Borneo, nature destroyed for oil palm plantations and construction.

Deforestation | Shutterstock

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.

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

  • 31 Aug 2015, 16:00
  • Robert McSweeney
Coral colony on a reef, Egypt

Tropical reef | Shutterstock

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.

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Scientists warn of unprecedented damage to forests across the world

  • 20 Aug 2015, 19:00
  • Robert McSweeney
Edge of the forest with dead trees

Forest edge | Shutterstock

Forests around the world are being affected by humans - both directly by deforestation and indirectly by climate change, say experts in a special issue of the journal Science.

In a series of reviews of the latest research into the health of the world's forests, scientists find they are far from being in the best shape for coping with climate change over this century. And this could affect how well trees absorb and store carbon in the future, they say.

Forest distribution

The world's forests generally fall into three categories, according to where they're found. You have the warm, humid conditions of tropical forests around the equator, the mild conditions enjoyed by temperate forests in the mid-latitudes, and the freezing cold of boreal forests in the North.

Forest -distribution

Global forest distribution. Credit: Nicolle Rager Fuller, National Science Foundation.

Today's special issue tackles each type in turn. We'll start with tropical forests, home to half of the world's species of plants and animals.

Human impacts such as logging and clearance for farmland and mining have left less than a quarter of tropical forests intact, say the authors of  one of the special issue papers. The remaining three-quarters are either fragmented or otherwise degraded.

The grey shaded areas in the map below show where forest has been cleared since the 1700s, and the red areas show recent hotspots for deforestation. But through the coming century, the threat of forest clearance will be "increasingly combined with the impacts of rapid climatic changes," the researchers say.

Disturbed -forests

Map of current and historical year-round ("evergreen") and seasonal tropical forest extent. Source: Lewis et al. (  2015)

Tropical

Climate change will have competing impacts on forests, lead author Dr Simon Lewis, from  University College London and the University of Leeds, tells Carbon Brief:

"One the one hand more carbon dioxide in the atmosphere is good for tree growth, increasing carbon stocks. On the other higher air temperatures and drought events tend to reduce tree growth, decreasing carbon stocks."

 

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Climate change set to fuel more "monster" El Niños, scientists warn

  • 17 Aug 2015, 17:00
  • Roz Pidcock
Flooding from November 2006 flood in Washington

Severe flooding | Wikipedia

The much-anticipated El Niño gaining strength in the Pacific is shaping up to be one of the biggest on record, scientists say. With a few months still to go before it reaches peak strength, many are speculating it could rival the record-breaking El Niño in 1997/8.

Today, a new review paper in Nature Climate Change suggests we can expect more of the same in future, with rising temperatures set to almost double the frequency of extreme El Niño events.

Supersizing

Every five years or so, a change in the winds causes a shift to warmer than normal sea surface temperatures in the equatorial Pacific Ocean - known as  El Niño.

Together with its cooler counterpart, La Niña, this is known as the El Niño Southern Oscillation (ENSO) and is responsible for most of the fluctuations in global weather we see from one year to the next.

Last week, US scientists  confirmed they expect "a strong El Niño" to peak in the next few months. The event brewing in the Pacific is already "significant and strengthening", said the statement from NOAA's Climate Prediction Centre.

The  latest temperature maps, released today, confirm parts of the tropical Pacific are up to 3C warmer than the long term average (dark red in the map below).

Average -sst -anomaliesSea surface temperature anomalies in the Tropical Pacific over the last four weeks. Source: Climate Prediction Center/NCEP,  NOAA

A separate comment piece in the same journal explains how scientists have been left scratching their heads over why El Niño has reemerged with such vigour after a false start last year.

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UK butterflies could suffer ‘widespread extinction’ by 2050, study warns

  • 10 Aug 2015, 16:00
  • Robert McSweeney
Speckled wood butterfly

Speckled wood butterfly | Jim Asher

Frequent droughts and habitat loss could push drought-sensitive butterflies in the UK to local extinction by the middle of the century, new research suggests.

Even in a very optimistic scenario, where habitat is improved, the likelihood of these butterflies surviving climate change drops to zero by 2100, if emissions stay very high.

The "alarming" results highlight the need to limit climate change by capping emissions, the lead author tells Carbon Brief.

Very hungry caterpillars

In the summer of 1995, the UK experienced an exceptionally dry summer, the most arid since records began in 1776. Among the impacts of the hot, dry season, the population of several species of butterfly collapsed, says Dr Tom Oliver, a researcher at the UK's Centre for Ecology and Hydrology.

Heatwaves and droughts affect adult butterflies, says Oliver, but caterpillars are even more sensitive to extreme weather, he explains to Carbon Brief:

"If their host plants dry out under prolonged severe drought then this can cause death or, at the very least, reduce the quality of their food so that they grow very slowly."

Populations of some species of butterfly took several years to recover to normal levels, says Oliver.

With droughts  likely to become more frequent as the climate warms, Oliver set about testing what effect this could have on butterfly numbers. Looking at data from the UK Butterfly Monitoring Service (UKBMS), they identified six species that tend to suffer a fall in number after dry years and suffered a major drop after the 1995 drought.

Selection Butterflies

The six species of butterfly considered in the study: a) Cabbage white, b) Small cabbage white, c) Ringlet, d) Green veined white, e) Speckled wood, and f) Large skipper. Source: Oliver et al. (2015)

The results of the study, published in Nature Climate Change, suggest that rising temperatures and fragmented habitats could mean we lose entire species of butterfly by the middle of the century.

Recovery time

With global temperature as it is today, we wouldn't expect another drought as severe as 1995 for more than 200 years. But climate change will drastically cut this return time in the future, the researchers say.

Even if global temperature rise is held at 2C above pre-industrial levels - that's  1.15C above the warming we've already seen to date - we could still expect to see a drought as serious as 1995 every six years. On the other hand, if emissions stay very high, a severe drought could be expected almost every year.

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Rising costs of flood defences could put world's major deltas at risk

  • 06 Aug 2015, 19:00
  • Robert McSweeney
The 2011 Mississippi flood. Credt: Mary/Flickr

Mississippi flood, 2011 | Mary/Flickr

Rising seas, sinking land and increasingly expensive flood defences could see the risk of flooding in deltas such as the Mississippi and Rhine rise eight-fold, a new study finds.

Researchers assessed the current and future flood risk in 48 deltas around the world and found that those in developed countries could face the biggest increase in risks if they can't maintain their investments in flood defences.

The Delta blues

Deltas form where rivers flow into the sea and deposit the sediment they're carrying. Some of the largest cities in the world are built on river deltas, from Cairo and Chittagong to Shanghai and San Francisco.

This sediment compacts naturally over time. So to keep the land surface from subsiding, the delta needs a regular top up, says Dr Zachary Tessler, a researcher at the City University of New York. He explains to Carbon Brief:

"In natural, or un-managed, deltas the land subsidence results in more river and coastal floods, which in turn distribute more sediment, rebuilding and maintaining the land surface elevation."

But human development can get in the way of this natural process, Tessler says:

"Human activities affect the integrity of deltas by reducing the amount of sediment in freshwater from the upstream river, and also reducing how effectively this sediment is deposited on the delta."

Building dams and reservoirs upstream of the delta holds back sediment, while using dikes and levees to control water levels can stop sediment from spreading onto the delta, Tessler says.Without adding new sediment, the land surface subsides, leaving deltas more at risk from flooding - both from the river that feeds it and the rising seas it flows into.

For cities built on river deltas, many have flood defences to protect them, but these need regular improvement and investment to keep a city protected.

Developed countries can typically afford to do this, but what if they couldn't? Tessler's study, published today in Science, asks that very question.

Risk and exposure

The researchers assessed the risk of flooding at 48 major coastal deltas across the world, home to around 340m people between them.

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Drought stunts tree growth for four years, study says

  • 30 Jul 2015, 19:00
  • Robert McSweeney
Stressed trees in US

Stressed trees in US | L Anderegg

 

Trees could take up to four years to return to normal growth rates in the aftermath of a severe drought, a new study finds. 

With the frequency and severity of droughts likely to increase with climate change, we might not be able to rely on forests to absorb as much of our carbon emissions, the researchers say.

Drought stress

Forests hold almost half of the carbon found on the Earth's surface, storing it in their woody trunks and branches.  Studies show that forests are sensitive to droughts, causing tress stress and limiting how much they can grow and store carbon.

During the European heatwave in 2003, for example, tree and plant growth  fell by 30%. That meant the land surface in Europe actually produced more carbon dioxide than it absorbed that year.

The new study, published in  Science, suggests that it takes longer for trees to recover after a severe drought than previously thought.

Tree rings

Using data from the International Tree Ring Data Bank, researchers analysed tree growth at over 1,300 sites across the northern hemisphere countries. The sites are predominantly in North America and Europe, and oak and pine trees make up the majority of the species the researchers considered.

Tree rings provide a handy estimate of how quickly a tree has grown. As a tree grows, it puts on extra layers of wood around its trunk, creating a new ring each year. The quicker a tree grows, the bigger the gap between tree rings from one year to the next. 

800px -tree _rings

Tree rings. Creative Commons 2.5: Arnoldius

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Global survey: Where in the world is most and least aware of climate change?

  • 27 Jul 2015, 17:35
  • Robert McSweeney

Form closeup | Shutterstock

Analysis of a global survey finds that more than a third of the world's adults have never heard of climate change. For some countries, such as South Africa, Bangladesh and Nigeria, this rises to more than two-thirds of the adult population.

The study says that education is the "single strongest predictor" of public awareness of climate change. Improving basic education and public understanding of climate change are vital to garner support for climate action, the researchers add.

Awareness and concern

The new study, published in Nature Climate Change, uses the results of a Gallup World Poll in 2007-08, which collected responses in 119 countries. This is the largest survey ever conducted on climate change, the paper's authors tell Carbon Brief, representing more than 90% of the world's population.

The poll asked people: "How much do you know about global warming or climate change?" Those who were aware of the issue were then asked the follow-up question: "How serious a threat is global warming to you and your family?"

The results show that adults in developed countries were more likely to say they are aware of climate change. Awareness rates in much of North America and Europe were well over 90% of respondents. Japan comes top with 99% of the population aware of climate change, with the US (98%) and Finland (98%) following closely behind.

Lee Et Al (2015) Table 1

Percentage of respondents saying they were aware of climate change: top and bottom 10 countries. Data source: Lee et al. (2015)

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Global risk of wildfires on the rise as the climate warms, study says

  • 14 Jul 2015, 16:00
  • Robert McSweeney

Bushfire | Shutterstock

An average of 3.5m square kilometres of land go up in smoke each year as a result of wildfires. Annual carbon dioxide released in these infernos can exceed half the emissions from humans burning fossil fuels.

A new study finds that the number of days wildfires are likely to burn each year is increasing as global temperatures rise.

Researchers estimate that between 1979 and 2013, the wildfire season has lengthened by an average of 19% for more than a quarter of the Earth's vegetated surface.

Weather and wildfires

From the 600 fires that claimed more than 50 lives in Russia in 2010, to the series of major bushfires in Australia in 2013 that caused $9.4bn of damage, wildfires affect much of the world's surface.

Wildfires play an important role in flammable ecosystems, such as forests, grasslands, savannas, and shrublands. They can be managed to disperse plants, clear forests and promote grazing, or suppressed to protect human lives and property.

Most wildfires are triggered by humans - as much as 90% in the US, for example - while natural causes include lightning and lava. But the weather is the biggest driver of how much area that wildfires actually burn. Temperature, humidity, rainfall and wind speed all play a role in providing the right conditions for a fire.

A new study, published in Nature Communications, finds that changes in these different weather variables are conspiring to increase the risks of wildfires.

Fire season length

Researchers analysed three global weather datasets to develop a metric for "fire weather season length" - the number of days per month where conditions create a high fire danger. They then worked out how the season length had changed between 1979 and 2013 for vegetated areas across the world. You can see the results in the graphs below.

Jolly Et Al 2015 Fig 2c

Graphs show a) Global average wildfire season length (expressed as a standardised anomaly), and b) Total global average area experiencing 'long' wildfire seasons (as a % of global vegetated area) - both from 1979 to 2013. Source: Jolly et al. (2015)

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How volcanic eruptions changed climate and human history

  • 08 Jul 2015, 18:00
  • Dr Joseph McConnell
Night eruption volcano Stromboli Glowing rocks falling down in Phase2

Erupting volcano | Shutterstock

A guest post from Dr Joseph McConnell, a research professor at the Desert Research Institute, which is part of the Nevada System of Higher Education in the US.

Throughout human history, large volcanic eruptions have affected the year-to-year variability of the  Earth's climate and even triggered crop failures and famines. These events may also have contributed to disease pandemics and the decline of agriculture-based societies.  

In our study published today in the journal Nature, we used ice-core records to provide a new reconstruction of the timing of nearly 300 individual volcanic eruptions extending as far back as the early Roman period. And then we worked out the radiative forcing of these eruptions - or how they have affected the energy balance of the Earth.

Summer cooling

When volcanoes erupt, they inject large amounts of sulphur dioxide into the upper atmosphere. These combine with oxygen and water to form  sulphate aerosols, which shield the Earth's surface from incoming solar radiation and cause cooler temperatures for as long as two years after an eruption.

We derived our reconstruction of past eruptions by looking for these aerosols in more than 20 individual ice cores extracted from ice sheets in Greenland and Antarctica.

These new records show that large volcanic eruptions in the tropics and high latitudes were the dominant drivers of climate variability, responsible for numerous and widespread summer cooling extremes during the past 2,500 years.

Our study shows that 15 of the 16 coldest summers recorded between 500 BC and 1,000 AD followed large volcanic eruptions - with four of the coldest occurring shortly after the largest volcanic events.

Our team of 24 scientists from the United States, United Kingdom, Switzerland, Germany, Denmark, and Sweden verified the timing of these events with the help of tree ring data. To align the two types of data, we used a distinctive signature of an extra-terrestrial cosmic ray event around 774-775 AD, which would could see in the tree rings and ice cores.

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