Analysis

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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Chance of a very cold UK winter falls to less than 1% by 2100, new study suggests

  • 06 Jul 2015, 16:00
  • Robert McSweeney

The odds of the UK having a winter as cold as the one in 2009-10 will drop to less than 1% by the end of the century as global temperature rise, researchers from the UK Met Office say.

The new research combines long-term projections of climate change with the yearly ups and downs of the UK's notoriously changeable weather. 

The results suggest that very cold winters and wet summers will become less and less likely, but the research says we should still expect them from time to time.

Cold snap

Climate projections tend to focus on how the typical weather of each season will change in the future. Yet British weather is often anything but typical.

Projections of climate change tell us that UK winters are likely to  get milder. Yet the winter of 2009-10 was the coldest in England and Wales for over 30 years, with average temperatures around  2C below the 1971-2000 average

This "apparent contradiction" led  some parts of the media to create confusion by claiming that our climate isn't warming at all, say the authors of a new study in Nature Climate Change.

But there isn't a contradiction at all, they say. And so they set out to show how a cold winter or a wet summer could still happen in a much warmer world - even if they are much less likely. 

Weather and climate

The researchers used the 'UK Climate Projections 2009' (UKCP09 ) - a set of projections of our future climate averaged into 30-year chunks, from 2010-2039, all the way to 2070-2099. They then combined them with model projections that are averaged over just a single year, and thus include more of the year-to-year variability in our weather. 

The resulting graphs, shown below, illustrate how different aspects of our weather are expected to change under a moderate emissions scenario. The black lines show observed changes and the red, blue and yellow lines show three (of many) different runs of their climate model. 

Annual Global Vs England Wales Temp

Observed (black lines) and projected (coloured lines) changes temperature/rainfall for individual seasons under the  A1B emissions scenario for England and Wales. Data show difference from the 1961-90 average. Source: Sexton and Harris (2015)

The results show how even with a clear climate change trend, we are still likely to be on the receiving end of some highs and lows of British weather, the researchers say.

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The most influential climate change papers of all time

  • 06 Jul 2015, 12:00
  • Roz Pidcock
Front covers of the most influential climate papers

Climate papers | Carbon Brief

Which of the many thousands of papers on climate change published each year in scientific journals are the most successful? Which ones have done the most to advance scientists' understanding, alter the course of climate change research, or inspire future generations?

On Wednesday, Carbon Brief will reveal the results of our analysis into which scientific papers on the topic of climate change are the most "cited". That means, how many times other scientists have mentioned them in their own published research. It's a pretty good measure of how much impact a paper has had in the science world.

But there are other ways to measure influence. Before we reveal the figures on the most-cited research, Carbon Brief has asked climate experts what they think are the most influential papers.

We asked all the coordinating lead authors, lead authors and review editors on the last Intergovernmental Panel on Climate Change (IPCC) report to nominate three papers from any time in history. This is the exact question we posed:

"What do you consider to be the three most influential papers in the field of climate change?"

As you might expect from a broad mix of physical scientists, economists, social scientists and policy experts, the nominations spanned a range of topics and historical periods, capturing some of the great climate pioneers and the very latest climate economics research.

Here's a link to our  summary of who said what. But one paper clearly takes the top spot.

Winner: Manabe & Wetherald (  1967)

With eight nominations, a seminal paper by Syukuro Manabe and Richard. T. Wetherald published in the Journal of the Atmospheric Sciences in 1967 tops the Carbon Brief poll as the IPCC scientists' top choice for the most influential climate change paper of all time.

Entitled, "Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity", the work was the first to represent the fundamental elements of the Earth's climate in a computer model, and to explore what doubling carbon dioxide (CO2) would do to global temperature.

Fig1  Manabe & Wetherald (1967), Journal of the Atmospheric Sciences

The Manabe & Wetherald paper is considered by many as a pioneering effort in the field of climate modelling, one that effectively opened the door to projecting future climate change. And the value of climate sensitivity is something climate scientists are  still grappling with today.

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Solar minimum could bring cold winters to Europe and US, but would not hold off climate change

  • 23 Jun 2015, 16:00
  • Robert McSweeney
Low winter sun over a common

Winter sun | Flickr

Over the past few decades, our Sun has been relatively active, giving off high levels of the solar radiation that warms the Earth. However, in recent years this peak activity has tailed off, prompting scientists to wonder if the Sun is heading into a period of lower output.

A new study says even if the Sun's activity did drop off for a while, it wouldn't have much impact on rising global temperatures. But it could mean a higher chance of a chilly winter in Europe and the US, the researchers say.

Solar output

The Sun's activity rises and falls on an approximately 11-year cycle, but it can experience longer variations from one century to another. Over the past 10,000 years, the Sun has hit around 30 periods of very high or very low activity - called 'grand maxima' and 'grand minima'.

One of these occurred between 1645 and 1715, when the Sun went through a prolonged spell of low solar activity, known as the Maunder Minimum. This didn't have much of an effect on global climate, but it was linked to a number of  very cold winters in Europe.

In 2010, scientists  predicted an 8% chance that we could return to Maunder Minimum conditions within the next 40 years.

But since that study was published, solar activity has declined further, and this likelihood has increased to 15 or 20%, says new research published today in open-access journal Nature Communications.

In fact, the Sun's output has declined faster than any time in our 9,300-year record, say the researchers. And so they set out to analyse what this could mean for global and regional climate.

Small decrease

The researchers used a climate model to run two scenarios where solar activity declines to a grand minimum. They then compared the results with a control scenario where the Sun continues on its regular cycle.

For all model runs they used the RCP8.5 scenario to account for future climate change - this is the scenario with the highest greenhouse gas emissions of those used by the Intergovernmental Panel on Climate Change ( IPCC). Global emissions are currently tracking just above this scenario.

You can see the modelling results in the maps below. Overall, a grand solar minimum could see global average temperature rise trimmed by around 0.12C for the second half of this century, the researchers say. Larger changes (shown as dark greens and blues) are seen in some parts of the northern hemisphere.Ineson Et Al (2015) Fig2Projected difference in annual average surface temperature for 2050-99 between RCP8.5 emissions scenario and a) Solar scenario 1 and b) Solar scenario 2. Areas of blue and green show regions projected to be cooler because of the solar minimum. Source: Ineson, S. et al. (2015)

 

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Tackling climate change will reap benefits for human health

  • 23 Jun 2015, 00:01
  • Roz Pidcock

Curbing climate change could be the biggest global health opportunity of the 21st century. But if we choose not to act, we could reverse all the progress made by economic development in the last 50 years towards improving global public health.

These are the conclusions of a new report by the Lancet Commission out today.

Curbing air pollution, phasing out coal, access to clean energy worldwide and promoting healthier lifestyles would have "immediate gains" for human health, says the report. 

The authors also call for a global price on carbon and a scaling-up of adaptation financing.

The Lancet Commission is a body set up to map out the impacts of climate change on health, and make recommendations to improve health standards worldwide.

Today's report is a collaboration between European and Chinese academics across the physical, health, political and social sciences, economics, energy policy and engineering.

Impacts are here and now

The risks posed by climate change are already unacceptably high, today's report begins:

"After only 0.85C warming, many anticipated threats have already become real-world impacts."

And if we continue to track the highest emissions scenarios - taking us to  4C or 5C by the end of the century - the risk of potentially catastrophic impacts rises even higher, the report adds.

Screenshot 2015-06-22 17.21.32

Changing exposure in over 65s to heatwaves by 2090 for RCP8.5 (left). Growth in annual heatwave exposure for over 65s with and without accounting for a growing and ageing population (right). Source: Lancet Commission report on health and climate change (2015)

The impacts of climate change on human health are all-pervading. Small risks can interact to produce larger-than-expected chances of catastrophic outcomes, the report notes.

As well as the  direct effects of rising temperatures on heat stress, floods, drought and other extreme weather, climate change increases air pollution, alters the spread of disease and raises the risk of food insecurity, malnutrition, migration and conflict.

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Climate change attribution studies are asking the wrong questions, study says

  • 22 Jun 2015, 16:30
  • Robert McSweeney
Devastation after cyclone Pam

Cyclone Pam | Wikimedia

Scientists are calling for a rethink in the way we seek to understand how climate change affects extreme weather.

The latest in so-called attribution studies is to study each  individual event by itself, looking for how climate change may have made it stronger or more likely.

But a new paper says the methods used in many of these studies underestimate the influence of climate change, and suggests a new approach to identify the "true likelihood of human influence".

Single-event attribution

One of the first studies to attribute a single extreme weather event to climate change was published just over a decade ago. Researchers  showed that climate change had doubled the chances of the record heatwave Europe experienced in 2003.

In the years that followed, many more studies have aimed to provide answers on how climate change is affecting our most brutal weather.

But while scientists have been able to attribute events caused by temperature extremes, linking other extreme events like storms and heavy rainfall events has proved more difficult, says a new paper in Nature Climate Change.

Canicule _Europe _2003Difference in temperature for 20 July to 20 August 2003 compared to long-term average. Source: Reto Stockli and Robert Simmon (NASA).

In our chaotic weather system, the complex dynamics of the atmosphere mean the size and path of a storm or heavy rainfall event has a large element of chance, the authors say. This can make it tricky to identify where climate change fits in.

But rather than analysing the wind patterns that bring a storm to an area, scientists should be looking at how the impact of that storm has been boosted by temperature changes -  known as thermodynamic effects.

Temperature increases mean more moisture evaporates into the atmosphere and more ice melts into our warming oceans, raising their levels. These are changes that scientists can be confident of, the authors say, and so should be the basis for attribution studies - rather than looking at changes to circulation patterns in the atmosphere.

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The Atlantic 'conveyor belt' and climate: 10 years of the RAPID project

  • 19 Jun 2015, 18:40
  • Roz Pidcock and Robert McSweeney

A global project that's been instrumental in shaping scientists' understanding of how the oceans affect our climate celebrated its tenth birthday recently.

A new paper published in  Science looks back at 10 years of the  RAPID project, which has been keeping tabs on how heat moves around in the Atlantic Ocean since 2004.

Over its short lifetime, the project has thrown up a few surprises. Parts of the Atlantic circulation seem to have slowed down, though whether that's down to human activity remains to be seen.

Carbon Brief talks to one of RAPID's founding scientists, Prof Harry Bryden from the National Oceanography Centre in Southampton, about the project's past and future.

Global heat transport

Above about 1,000 metres in the North Atlantic, warm water flows northwards from the equator towards the poles, releasing heat as it goes. The water cools and sinks at high latitudes, returning southwards towards the equator at much deeper depths.

This is known as the Atlantic Meridional Overturning Circulation (AMOC) and forms part of a global ocean conveyor belt that transports heat all around the world.

                 Screenshot 2015-06-19 18.22.21

The Atlantic Meridional Overturning Circulation (AMOC). Warm water flows north in the upper ocean (red arrows) then sinks and returns south as deep cold water (blue arrows) Source: Srokosz & Bryden (  2015) Supplementary material

The Gulf Stream - another component of the AMOC - is driven by the wind. Heat released to the atmosphere as the warm Gulf Stream moves northward gives northwest Europe its mild climate.

All components of the AMOC together transport about 18 million cubic metres of water per second - equivalent to a hundred times the flow from the Amazon river. The heat carried with it means North Atlantic sea surface temperature is about  5C warmer than in the North Pacific at similar latitudes.

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