The foliage on native plants in Australia's arid zones is boosted by CO2

Increased levels of carbon dioxide (CO2) have helped boost green foliage across the world’s arid regions over the past 30 years through a process called CO2 fertilisation, according to CSIRO research.

In findings based on satellite observations, CSIRO, in collaboration with the Australian National University (ANU), found that this CO2 fertilisation correlated with an 11 per cent increase in foliage cover from 1982-2010 across parts of the arid areas studied in Australia, North America, the Middle East and Africa, according to CSIRO research scientist, Dr Randall Donohue.

"In Australia, our native vegetation is superbly adapted to surviving in arid environments and it consequently uses water very efficiently," Dr Donohue said. "Australian vegetation seems quite sensitive to CO2 fertilisation.

The fertilisation effect occurs where elevated CO2 enables a leaf during photosynthesis, the process by which green plants convert sunlight into sugar, to extract more carbon from the air or lose less water to the air, or both.

This, along with the vast extents of arid landscapes, means Australia featured prominently in our results."

"While a CO2 effect on foliage response has long been speculated, until now it has been difficult to demonstrate," according to Dr Donohue.

"Our work was able to tease-out the CO2 fertilisation effect by using mathematical modelling together with satellite data adjusted to take out the observed effects of other influences such as precipitation, air temperature, the amount of light, and land-use changes."

The fertilisation effect occurs where elevated CO2 enables a leaf during photosynthesis, the process by which green plants convert sunlight into sugar, to extract more carbon from the air or lose less water to the air, or both.

If elevated CO2 causes the water use of individual leaves to drop, plants in arid environments will respond by increasing their total numbers of leaves. These changes in leaf cover can be detected by satellite, particularly in deserts and savannas where the cover is less complete than in wet locations, according to Dr Donohue.

"On the face of it, elevated CO2 boosting the foliage in dry country is good news and could assist forestry and agriculture in such areas; however there will be secondary effects that are likely to influence water availability, the carbon cycle, fire regimes and biodiversity, for example," Dr Donohue said.

"Ongoing research is required if we are to fully comprehend the potential extent and severity of such secondary effects."

This study was published in the US Geophysical Research Letters journal and was funded by CSIRO's Sustainable Agriculture Flagship, Water for a Healthy Country Flagship, the Australian Research Council and Land & Water Australia.

Reprinted from CSIRO

Donohue, R. J., M. L. Roderick, T. R. McVicar, G. D. Farquhar. 2013 "Impact of CO₂ fertilization on maximum foliage cover across the globe's warm, arid environments. Geophys. Res. Lett. 40, doi:10.1002/grl.50563

Non-deciduous perennials are responding better than deciduous, annual and ephemeral plants

Here is the abstract from a previous paper by Dr Donohue (my paras):

Using Advanced Very High Resolution Radiometer data spanning 1981–2006 and calibrated for long-term analyses of vegetation dynamics, we examine whether vegetation cover has increased across Australia and whether there has been a differential response of vegetation functional types in response to changes in climatic growing conditions. Trends in vegetation cover are interpreted within Budyko's energy – water limitation framework.

Results from an Australia-wide analysis indicate that vegetation cover (as described by the fraction of Photosynthetically Active Radiation absorbed by vegetation; fPAR) has increased, on average, by 0.0007 per year – an increase of ∼8% over the 26 years. The majority of this change is due to a 0.0010 per year increase in persistent fPAR (representing nondeciduous perennial vegetation types; up 21%).

In contrast, recurrent fPAR (representing deciduous, annual and ephemeral vegetation types) decreased, on average, by 0.0003 per year (down 7%), the trends of which are highly seasonal.

Over the same period, Australian average annual precipitation increased by 1.3 mm yr−2 (up 7%).

A site-based analysis using 90 long-term meteorological stations with minimal localized land-cover changes showed that energy-limited sites where total fPAR increased generally experienced decreases in precipitation, and water-limited sites that experienced decreases in cover were almost always associated with decreases in precipitation.

Interestingly, where vegetation cover increased at water-limited sites, precipitation trends were variable indicating that this is not the only factor driving vegetation response. As Australia is a generally highly water-limited environment, these findings indicate that the effective availability of water to plants has increased on average over the study period. Results also show that persistent vegetation types have benefited more than recurrent types from recent changes in growing conditions.

Regardless of what has been driving these changes, the overall response of vegetation over the past 2–3 decades has resulted in an observable greening of the driest inhabited continent on Earth.

For how long does CO2 stay up in the air?

This article in Nature discusses the length of time it's likely that CO2 stays up in the air. It's a very long time. Some of it stays around for thousands and thousands of years.

Some people find it hard to understand why CO2 makes a difference to warming. (I've already outlined a simple description of why the earth is warming.)

CO2 warms the earth because of its greenhouse properties. Greenhouse gases are the ones that stop the earth from turning into an ice block. They keep us nice and warm so that we can grow crops, go swimming and do a host of things that would be impossible if the earth was too cold.

Because we keep adding CO2 to the atmosphere by digging up fossil fuels and removing trees, the earth is getting warmer. Because the earth is getting warmer, the amount of water vapour in the air increases. Water vapour in the air can't be seen. It's just another gas.

When water vapour condenses, it forms clouds and we can certainly see clouds but that's no longer water vapour. Clouds are made up of drops of liquid water or ice. Clouds don't have a greenhouse effect as such. Some of them cool the earth and some of them warm the earth. On balance, they don't make much difference to the temperature overall.

Water vapour, on the other hand, is another greenhouse gas so it also makes the earth warmer. That means that now there are two gases that have increased in the air, making the earth warmer - the extra water vapour plus the extra CO2. So the earth gets even warmer than if only one of these greenhouse gases was increasing.

If the earth wasn't getting warmer, the amount of water vapour on average wouldn't change. In fact, water vapour recycles through the air every few days. So if we could cool the earth a bit the amount of water vapour would lessen and the temperature of the earth would drop back to what it used to be like a few years ago.

Unfortunately, it's not easy to cool the earth. One way would be to soak up the extra CO2 in the air. Some scientists are working on how this could be done. But at the moment even if it could be done it's expensive and could have unwanted effects. In any case, we'd have to soak up more than we're putting up in the air to make any difference.

That's why, for now, the best thing to do is to switch over to renewable energy sources as soon as we can, and stop digging up fossil fuels.

For more information on climate change, check out the links on this site.

A highly simplified view of global warming

To change the climate of the entire earth requires a change in the balance of radiation.

Energy in = energy out - stable climate
Energy in less than energy out - earth gets cooler (ice ages)
Energy in more than energy out - earth gets warmer (interglacials)

Temperature is one way of measuring energy. The same object measured at a higher temperature has more energy than that object when it's cooler.

When as much energy goes out of the earth as comes into the earth, there won't be a global temperature change. There will continue to be changes in climate in different parts of earth but it will balance out over the whole earth.

The temperature of the whole earth is rising and has been more or less doing so since about the time the industrial revolution took off. This rise in temperature can in essence be only from one of two causes. Either there is more energy coming in from the sun or there is less energy going back out to space from the earth. (If the earth was cooling it would be the reverse.)

The reason the earth is warming is that the earth is holding in more energy than is going out into space. Since the middle of the 20th century, measurements of the sun's energy and the earth's current orbit and the way it is aligned to the sun show that the earth has not been receiving more energy from the sun. (Before this time, some of the extra heat can be attributed to there being more energy coming in from the sun as well as the extra CO2.)

The greenhouse effect describes how earth retains heat to support life on earth. The recent rise in temperature is mainly caused by the large rise in one of the greenhouse gases, CO2, which means less energy is being radiated out, which means the earth is getting warmer. As a result of this initial warming, there are other effects that all add to the change - water vapour, which is another greenhouse gas, increases and lets even less energy escape, ice reduces and means less energy is reflected out to space, and so on.

Eventually, the energy state will re-balance. The temperature will stabilise at a new higher level and the amount of energy coming in will again equal the amount of energy going out.

An analogy of the adjustment to the change in energy balance is - if you turn on a tap over the sink and allow the water to drain out the plughole. If you partly plug up the drain then water will start to rise in the sink. There will be more water coming in from the tap than going down the drain. This is the same effect as adding more CO2 blocks some of the heat from leaving the earth, and the energy increases and temperature rises. For a while the water will continue to rise in the sink (similar to the temperature going up) until it reaches a point where the water coming out of the tap equals the water going down the drain. The pressure of the water in the sink will cause more water to go down the drain and equalise with the water coming out of the tap. The water level will be higher when it equalises, just like the temperature will be higher when the earth's energy balance gets to a new equilibrium.

The above description is a very simplistic description of the situation. Much more detail is available from reputable sources closer to the science itself. Check the links here for a more detailed and scientific explanation and other related information.