Climate science

Emissions cuts using biofuels could worsen water stress in US, study suggests

  • 03 Aug 2015, 20:00
  • Robert McSweeney

Using biofuels as a way to cut greenhouse gas emissions could put US water resources under increasing pressure, a new study suggests. Researchers find that a heavy reliance on bioenergy could mean a 42% increase in water consumption across the US by 2100.

The results show that policies to mitigate climate change need to be carefully planned to avoid knock-on effects on natural resources, the researchers say.

Water deficits

The new study, published in Proceedings of the National Academy of Sciences, analysed "water deficits" across the US. Deficits occur when demand for water outstrips the supplies available in local or neighbouring rivers and reservoirs.

The map below shows that much of the US is already in deficit for water. The orange and red areas in the main map below show the counties in US that are already worst affected by water stress.

Deficit Over Demand MapAverage total annual water deficit as a fraction of demand in 2005. Source: Hejazi et al (2015)

Using computer models, the researchers simulated how these deficits might change with different approaches for tackling global carbon emissions to limit the impacts of climate change.

The results show that measures to cut carbon emissions could mean water deficits are more severe by the end of the century.

Bioenergy

The study compared the impact on water stress of two scenarios, or representative concentration pathways, for how we deal with rising carbon emissions.  RCP4.5 describes a world where the concentration of carbon dioxide in the atmosphere is stabilised at around 650 parts per million (ppm) by 2100.

In contrast, RCP8.5 is a future where emissions are not curbed and carbon dioxide concentrations rise to around 1,370ppm by 2100. The paper refers to RCP8.5 as a "business-as-usual" scenario.

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Two degree climate target not possible without 'negative emissions', scientists warn

  • 03 Aug 2015, 17:05
  • Roz Pidcock

All of our options for keeping warming below 2C above pre-industrial temperatures now involve capturing carbon dioxide and storing it underground - a technology that doesn't yet exist on a large scale, according to new research.

The study, published today in Nature Communications, argues that 'negative emissions' alone, in the absence of conventional mitigation, are unlikely to achieve the 2C goal.

And in all but the most optimistic cases, staying below 2C requires capturing and storing carbon in amounts that exceed the capabilities of current technology, say the researchers.

Ahead of a major international climate summit in Paris, the study makes an interesting contribution to the debate about the role of negative emissions in meeting the 2C target.

Budgets

In December, global leaders will gather in Paris to agree a deal for capping global temperature rise at 2C above pre-industrial levels - the internationally agreed target.

For any given temperature target, there is a finite amount of carbon that can be burned before the chances of staying below that target become minimal. This is known as a  carbon budget.

In its latest report, the Intergovernmental Panel on Climate Change (IPCC) said that to have a reasonable chance of staying below 2C, total emissions from all human activity must not exceed 1,000bn tonnes of carbon (or gigatonnes of carbon, GtC).

The world is currently not on course to meet this target and there are two options for how to get ourselves back on track, says today's paper.

The first is to produce fewer emissions, which means burning fewer fossil fuels. This is what's commonly referred to as conventional mitigation.

The other is to capture fossil fuel emissions before they enter the atmosphere, or to suck them directly out of the air - a technique known as carbon dioxide removal. A third possibility sometimes proposed is to artificially engineering parts of the climate system, such as the oceans, to take up more carbon.

Collectively, the new paper calls these "negative emissions" technologies.

(Note that this is not shorthand for what the IPCC refers to as "net negative emissions". In its latest report, the IPCC  said: "Net negative emissions can be achieved when more GHGs are sequestered than are released into the atmosphere (e.g., by using bio-energy in combination with carbon dioxide capture and storage).

Carbon Sequestration

Laboratory studies help progress the field of carbon storage, by characterising the chemical signatures that result when certain types of rocks are exposed to carbon dioxide. Credit: Idaho National Laboratory | Flickr

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New study shrinks the gap between observed and modelled global temperatures

  • 31 Jul 2015, 16:15
  • Roz Pidcock

It's well known in climate science that global surface temperatures over the past decade have been lower than climate models expected them to be.

Some parts of the media have jumped on this to suggest climate models  overestimate the amount of warming we can expect in future.

Now, a   new paper says the discrepancy between modelled and observed temperatures isn't as large as previously thought.

Taking into account the different ways they estimate global temperature shrinks the difference between them by more than a third, according to the study in Geophysical Research Letters.

Model mismatch

While heat-trapping greenhouse gases in the atmosphere have continued to rise, recent measurements of surface temperatures suggest the world is warming a little more slowly than models projected.

The black line in the top graph below shows how the observed temperature from  HadCRUT4 is currently tracking the lower end of the range expected by climate models used in the last Intergovernmental Panel on Climate Change report (red line and purple shading).

We'll come back to the bottom graph in a minute.

Forcings -graph

Top: Comparison of 84 model simulations of the IPCC's highest emissions scenario (RCP8.5) against HadCRUT4 observations (black), using air temperatures (red line and shading) or "blended" temperatures (blue line and shading). Coloured lines are the multi-model averages, shading is the 5-95% uncertainty range of the model simulations. Bottom: Results adjusted to include updated aerosol forcings from Schmidt et al. [2014]. Anomalies are relative to 1961-1990. Source: Ed Hawkins,  Climate Lab Book

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