Based on current greenhouse gas emissions, the world is on track for 4C warming by 2100 – well beyond the internationally agreed guardrail of 2C. To keep warming below 2C, we need to either reduce our emissions, or take carbon dioxide out of the atmosphere.
Two papers published today investigate our ability to limit global warming and reverse the impacts of climate change. The first, published in Nature Communications, shows that to limit warming below 2C we will have to remove some carbon from the atmosphere, no matter how strongly we reduce emissions.
The second, in Nature Climate Change, shows that even if we can remove enough CO2 to keep warming below 2C, it would not restore the oceans to the state they were in before we began altering the atmosphere.
How we’re tracking
Currently, we’re at 400 parts per million – rising from 280 ppm before the industrial revolution.
To project future climate change the Intergovernmental Panel on Climate Change (IPCC) uses a range of emissions scenarios called Representative Concentration Pathways (RCPs), based on different economic and energy use assumptions.
In the high scenario, RCP8.5, emissions continue to grow from our present rate of 37 billion tonnes of CO2 per year to about 100 billion tonnes of CO2 in 2100, when atmospheric CO2 levels are projected to be 950 ppm. This scenario assumes little mitigation of our carbon emissions.
In the low scenario, RCP2.6, emissions rise slowly till the end of this decade to about 40 billion tonnes CO2 each year and then start to decline. Amongst the IPCC emission scenarios, only the RCP 2.6 appears capable of limiting warming to below 2C. With RCP 2.6 at the end of the century atmospheric concentrations is about 420 ppm, and only 20 ppm above the present value.
Present emissions are tracking close to the highest scenario (RCP8.5). If we want to keep warming below 2C it requires a substantial reduction in the amount of CO2 released into the atmosphere.
What we have to do
We have two options by which to reduce emissions, the first through reducing the use of fossil fuel energy, and the second through Carbon Dioxide Removal (CDR).
CDR refers to technologies that remove CO2, the primary greenhouse gas, from the atmosphere. Examples include Biomass Energy with Carbon Capture and Storage (BECCS), afforestation (planting trees), adding iron to the ocean, and directly capturing CO2 from the air.
For many CDR technologies the boundary between “climate intervention” (or “geoengineering”) and greenhouse gas mitigation is unclear. However, the goal is the same, enhancing the CO2 current taken up and sequestered by the land and ocean.
Can we just remove carbon?
The first study, led by Thomas Gasser, used results from 11 Earth System Models, in conjunction with a simple carbon-cycle models to simulate different emissions reductions scenarios associated with the low emissions pathway, RCP2.6.
They showed that under all emissions reductions scenarios, even slashing emissions to less than 4 billion tonnes CO2 each year, (greater than a 90% cut in current emissions) is insufficient to limit warming to 2C.
This means that some form of CDR will be required to keep warming at less than 2C. The exact level of CDR required depends very much on the emissions reduction achieved, from 2 billion to 10 billion tonnes of CO2 each year in the most optimistic scenario, to between 25-40 billion tonnes CO2 each year in the lowest emission reduction case. This is equivalent to current total global emissions.
The study also suggests that the requirements for CDR may indeed be even higher if unanticipated natural carbon cycle (positive) feedbacks were to occur. We may desire the ability to remove more carbon from the atmosphere to compensate for these.
The other study, led by Sabine Mathesius, explores whether CDR under high CO2 emissions can achieve a similar environmental outcome to a rapid transition to a low carbon energy use (RCP2.6).
It shows that aggressive CDR can only undo the effect of high emissions (RCP8.5) and return the marine environment to either pre-industrial values or the low emission scenario over thousands of years. The ability to undo the damage caused by high emissions reflects timescale of the ocean carbon cycle. While the upper ocean quickly reaches equilibrium with the atmosphere, the deeper ocean takes millennia to restabilise.
Such irreversibility of the system is an important consequence and the study provides valuable information to consider as we tackle rising CO2 levels. Both studies are theoretical but they provide an important perspective on the ability of mankind to engineer the climate system and undo the effects of high CO2 levels in the atmosphere.
No CDR or suite of CDR technologies exists capable of removing the levels of CO2 at the upper range of what maybe required. This means that, while CDR could aid in limiting global temperatures below 2C, in practice this is not even yet possible, and would not be without risks. This continues to be a very active area of research.
While the focus of both studies explore reversing the environmental changes of rising CO2, the climate system is complex and the possibility that mitigation options like CDR could produce unforeseen impacts is high. While reducing carbon emissions is the safest and preferred path for avoiding dangerous climate change and ocean acidification, it is likely that some CDR will be required to achieve this.
The authors will be one hand for an Author Q&A on Tuesday, August 4 – Andrew between 3 and 4pm AEST and Richard between 5 and 6pm AEST. Post your questions in the comments section below.
Changing wildlife: this article is part of a series looking at how key species such as bees, insects and fish respond to environmental change, and what this means for the rest of the planet.
As the world warms, animals and plants will shift their ranges to keep pace with their favoured climate. While the changing distributions of species can tell us how climate change is affecting the natural world, it may also have a direct impact on us.
One good example is the disease carried by insects.
Those small, familiar flies called mosquitoes are responsible for much human suffering around the globe because of their ability to transmit diseases.
Could climate change cause these diseases to spread? While this an extremely important health question, the answer is far from simple.
Complicated life cycle
The life cycle of mosquitoes and its viral parasites is particularly complicated.
Only adult females consume blood, and the immature stages (larvae) live in fresh or brackish water, filtering out small organic particles.
The virus undergoes certain parts of its lifecycle inside particular mosquito organs, but also requires other organs in the vertebrate host to complete its life cycle. And to get into a vertebrate, such as us, it relies on a hungry blood-sucking insect.
These viruses always have other hosts besides humans, which may include native and domestic animals. The pathway that these viruses take to infect humans is often via our domestic animals, which are also bitten by the same mosquitoes that feed on us.
In addition, rates of virus transmission to humans is also affected by the human built environment, and also human behaviour.
Because mosquitoes breed in water, changes in rainfall patterns are likely to change the distribution and abundance of mosquitoes, and therefore could affect disease transmission.
Australian climate is characterised by its variability, however we have experienced a general trend towards increased spring and summer monsoonal rain across northern Australia, and decreased late autumn and winter rainfall in the south.
Kunjin virus is mainly transmitted by a small mosquito called Culex annulirostris, the common banded mosquito, in Australia. We are lucky because human infection rarely causes disease, even though Kunjin and the common-banded mosquito are widespread in Australia.
Kunjin’s close relative, the US strain of West Nile Virus is much more virulent, causing more human disease. These viruses are well known for their ability to mutate quickly, so they are always keeping medical authorities on their toes.
Higher than average rainfall and flooding in eastern Australia in the second half of 2010 and 2011 provided ideal conditions for breeding common banded mosquitoes, and in 2011 a dangerous strain of Kunjin appeared that caused acute encephalitis (swelling of the brain) in horses. This disease has only been detected in one human, however this mosquito feeds on both humans and horses.
This new virulent strain of Kunjin also appeared in new areas east of the Great Dividing Range, suggesting other unknown changes in transmission.
As temperatures increase, mosquito activity will begin earlier in the season and reach higher levels of abundance sooner, and maintain higher populations longer. These factors will all probably tend to increase the rate of transmission of Kunjin to both humans and animals.
While flooding may have helped spread Kunjin, drought may have helped another mosquito-borne virus.
It would be simple to assume that drought would reduce mosquito populations by reducing the larval habitat (water), and thereby reduce the incidence of mosquito-borne disease in Australia.
However, this is not necessarily the case. Another Australian mosquito, Aedes notoscriptus, the striped mosquito, is responsible for transmitting Ross River and Barmah Forest Virus in Australia.
The striped mosquito is unusual in comparison to its cousins because it breeds in small containers of water, such as tree holes in natural environments. The main carrier of Dengue in Australia, Aedes aegypti, shares this habit.
These small container habitats abound in Australia’s urban backyard, with water features, water and food bowls for pets, and various toys providing such breeding places.
With the drought, Australians became much more water wise, and installed various water storage devices in their gardens, ranging from buckets left out in a storm, to professionally installed rain tanks. All these are potential habitat for the striped mosquito to breed.
In this case drought has caused an increase in the abundance of a mosquito virus carrier because of a change in human behaviour.
The return of Dengue?
Dengue fever is transmitted in Australia by the mosquito Aedes aegypti. The mosquito is restricted to Queensland, and Dengue fever transmission is restricted to coastal northern Queensland.
Recent modelling predicts that moderate climate change would extend the Dengue risk zone to Brisbane, exposing much larger human populations to risk.
However, before the 1930s, Dengue fever transmission was known south almost to Sydney, and Aedes aegypti was known throughout mainland Australia except the deserts.
Both the mosquito, and the disease, have retreated to Queensland since then, and we don’t know why. What is clear is that we don’t really understand what controls the distribution of Aedes aegypti or Dengue in Australia, but given the contraction of the disease in historical time, it is unlikely that a warming climate will produce a simple response in the insect or the disease.
Australian insects will be affected by climate change, but simple predictions based on increasing average temperatures and changing rainfall patterns miss the important effects of complex biological interactions.
In addition, we are only just beginning to use models that are sophisticated enough to consider how insects might evolve under changing climate.
Investing in a deeper understanding of these complex biological webs, and their outcomes for human society, will result in great returns. Our predictions of the future state of Australian plants and animals will become more accurate and we will also improve human health and manage our biodiversity more sustainably into the future.
Climate change and the loss of biodiversity are two of the greatest environmental issues of our time. Is it possible to address both of those problems at once?
In Australia, farmers and landholders will this week be able to apply for payments through the Federal government’s A$2.55 billion Emissions Reduction Fund. Bidders can request funding for projects that reduce emissions using agreed methods, which include approaches relevant to the transport, waste and mining sectors, as well as the land sector: for example, by managing or restoring forests.
Forests hold carbon in vegetation and soils and provide important habitat for native wildlife. Restoring forests in areas where they have been cleared in the past could be good for the climate, good for biodiversity, and generate additional income for landholders.
How well the Emissions Reduction Fund can achieve these benefits will depend on three things: the right approach, the right price, and the right location.
There are a range of approaches available for restoring forests, and they vary in how quickly carbon can be sequestered, cost, and suitability for wildlife.
For example, fast-growing monocultures such as blue gum plantations can sequester carbon very rapidly, but don’t provide ideal habitat for wildlife. Planting a diversity of native trees and shrubs using an approach called environmental plantings is far more wildlife-friendly, but the costs are higher, and carbon is not stored as quickly.
A third possible approach is to assist the natural regeneration of vegetation. This can be done by fencing off cattle or by ceasing on-farm practises such as burning or disturbance with machinery. Assisted natural regeneration is the cheapest of these three possible methods, and is also good for biodiversity: our recent paper found that it could be a great option for restoring forests in agricultural landscapes across Queensland and northern New South Wales.
Location, location, location
Across Australia, there are a number of places where growing carbon could be a more profitable option than the current land use. Some of these places are more important for biodiversity than others.
If we’re interested in getting some wins for biodiversity while growing carbon forests, we need to think carefully about the possible opportunities and trade-offs, as the best places for sequestering carbon are not always the most beneficial for biodiversity, and vice versa.
In our recent paper, we found that it is possible to identify where growing forests could provide win-wins for both carbon and biodiversity.
For example, the top 25% of priority areas for environmental plantings could sequester 132 million tonnes of CO2 equivalent annually, which is almost a quarter of Australia’s annual emissions (excluding those caused by land-use change).
These high-priority areas for environmental plantings could restore some of the most threatened ecosystems in Australia. There are 139 ecosystem types across the country that have lost more than 70% of their original extent. If it were possible to restore these ecosystems up to 30% of their original extent, they will have a better chance of surviving in the long term.
Restoring parts of the landscape with these ecosystems is a high priority for biodiversity – not only are the ecosystems rare, but many of the birds and animals that depend on these ecosystems are those that are most threatened. For example the brigalow woodlands of south east Queensland, of which less than 10% remain, are home to nationally threatened koalas and a host of other wildlife.
The right price
It will generally be more expensive to grow carbon forests that also provide benefits for biodiversity. This is because the places most profitable for land uses such as agriculture are often where the most threatened species and ecosystems are located.
In our analysis, we found that with a price on carbon equivalent to A$5 per tonne, it would not be profitable to restore threatened ecosystems up to 30% of their original extent. This means that without additional funding from another source, there is limited opportunity to achieve wins for biodiversity if the price on carbon is low.
However, a higher price of A$20 per tonne, reflecting Australia’s 2011-2013 carbon price, could allow up to half of the heavily cleared vegetation types to be restored up to 30% without any additional funding for biodiversity itself. At this A$20 price, we also found that it made more economic sense to farm carbon than the existing land use, in over 1.2 million hectares in Queensland.
This week’s Emissions Reduction Fund auction will be a good first test of how the current approach to carbon farming can provide the dual benefit of restoring habitat for native wildlife and addressing climate change. Our analysis shows that Australia’s climate policies could have a very significant impact on biodiversity – if we think carefully about the right approach, price, and location.
By Chris Gerbing
We all have an interest in whether rain will dampen our day and a curiosity about what the skies hold for next week. We are all impacted when the weather turns extreme, sometimes in devastating ways. And we have a yearning to know what the future might hold for our climate, so that we can plan ahead.
Weather and climate may never be completely predictable, but science has come far enough for us to be breaking new ground when it comes to projecting what Australia’s climate may look like decades – or even hundreds of years – in the future.
And here’s a sneak peak into the future – by the year 2090, Sydney could have the climate of Brisbane, and Melbourne could have the climate of Dubbo.
Climate models help us to understand our present weather and climate, and also allow us to consider plausible future scenarios of how the climate might change. Climate models are built using mathematical representations of the dynamic Earth system. Their fundamentals are based on the laws of physics including conservation of mass, energy and momentum. They create simulations to tell us what happened or what might happen under a range of different scenarios (such as greenhouse gas concentrations).
Check out this animation about climate models.
Along with the Bureau of Meteorology, we’ve used as many as 40 climate models, produced by international global climate modelling groups, to create projections for Australia’s climate, all the way out to the year 2090. The projections consider up to 15 regions of Australia, a small set of plausible future greenhouse gas scenarios and four future time periods.
Climate change projections are presented as a range of possibilities. This occurs because different models produce different projections. Even though they are based on the same physical laws, such as conservation of mass, moisture and energy, each climate model treats regional processes in the oceans and atmosphere slightly differently. It is important to explore the full range of possibilities in any impact assessment.
Even if we significantly reduce our greenhouse gas emissions as under an intermediate scenario, Melbourne’s annual average climate could look more like that of Adelaide’s, and Adelaide’s climate could be more like that of Griffith in New South Wales.
Eastern Australian coastal sites could see a climate shift to those currently typical of locations hundreds of kilometres north along the coast. Sydney’s climate could resemble that of Port Macquarie, and Coffs Harbour’s climate resembling that of the Gold Coast (by 2050; intermediate emissions).
This research received funding from the Department of Environment under the Regional Natural Resource Management Planning for Climate Change Fund. Additional funding was provided by CSIRO and the Bureau of Meteorology.
We have published a few articles over on The Conversation which takes a deeper look into the details of these climate models and projections.
- A new website shows how global warming could change your town
- Warmer, wetter, hotter, drier? How to choose between climate futures
- Explainer: The models that help us predict climate change
By Simon Torok
Tropical cyclones are an ongoing threat during Australia’s cyclone season, which generally lasts from November to April. On average, the Australian region experiences 13 cyclones a year.
But as the coastlines of Queensland and the Northern Territory are threatened on two simultaneous fronts (Marcia and Lam), we’ve asked our climate scientists what we can expect from tropical cyclones in the future, as Australia’s climate continues to change.
1. Has the frequency of tropical cyclones changed?
Some scientific studies suggest no change and others suggest a decrease in numbers since the 1970s in the frequency and intensity of tropical cyclones in the Australian region.
The Bureau of Meteorology’s satellite record is short and there have been changes in the historical methods of analysis. Combined with the high variability in tropical cyclone numbers, this means it is difficult to draw conclusions regarding changes.
However, it is clear that sea surface temperatures off the northern Australian coast have increased, part of a significant warming of the oceans that has been observed in the past 50 years due to increases in greenhouse gases. Warmer oceans tend to increase the amount of moisture that gets transported from the ocean to the atmosphere, and a warmer atmosphere can hold more moisture and so have greater potential for intense rainfall events.
2. Will the frequency of tropical cyclones change in future?
The underlying warming trend of oceans around the world, which is linked to human-induced climate change, will tend to increase the risk of extreme rainfall events in the short to medium term. Studies in the Australian region point to a potential long-term decrease in the number of tropical cyclones each year in future, on average.
On the other hand, there is a projected increase in their intensity. In other words, we may have fewer cyclones but the ones we do have will be stronger. So there would be a likely increase in the proportion of tropical cyclones in the more intense categories (category 4 or 5). However, confidence in tropical cyclone projections is low.
3. What are the impacts of tropical cyclones?
Today, coastal flooding is caused by storm tides, which occur when low-pressure weather systems, cyclones, or storm winds elevate sea levels to produce a storm surge, which combines with high or king tides to drive sea water onshore. Although rare, extreme flooding events can lead to large loss of life, as was the case in 1899 when 400 people died as a result of a cyclonic storm surge in Bathurst Bay, Queensland.
4. How will impacts of tropical cyclones change in future?
With an increase in cyclone intensity, there is likely to be an increased risk of coastal flooding, especially in low-lying areas exposed to cyclones and storm surges. For example, the area of Cairns’ risk of flooding, by a 1-in-100-year storm surge, is likely to more than double by the middle of this century.
5. How can we adapt to expected changes?
Almost all of our existing coastal buildings and infrastructure were constructed under planning rules that did not factor in the impacts of climate change. However, governments are now taking account of changes in climate and sea level through their planning policies. Just as the building codes and rules for Darwin changed in the wake of Cyclone Tracy, so they should now be re-assessed for each region and locality in Australia to take account of climate change.
You can track both Tropical Cyclone Marcia and Lam using our Emergency Response Intelligence Capability tool (ERIC).
And we also have more information about our latest climate projections here.
Australia is on track for up to 1.7C of warming this century if the world curbs its greenhouse emissions, but under a worst-case scenario could see anything from 2.8C to 5.1C of warming by 2090, according to new climate change projections released by the CSIRO and the Bureau of Meteorology.
The projections are the most comprehensive ever released for Australia. They are similar to those published in 2007, but based on stronger evidence, with more regional detail. These projections have been undertaken primarily to inform the natural resources management sector, although the information will be useful for planning and managing the impacts of climate change in other sectors.
The new report draws on climate model data used by the Intergovernmental Panel on Climate Change (the IPCC). The Fifth IPCC Assessment Report (AR5), released in 2013 and 2014, used a range various greenhouse gas and aerosol scenarios to project future climate change.
Over the past 10 years, carbon dioxide emissions have been tracking the highest IPCC emission scenario (known as RCP8.5). If there is limited international action to reduce emissions, then projections based on the highest scenario may be realised.
However, if emissions are significantly reduced over the coming decades, then intermediate emissions (RCP4.5) might be feasible. Following the low emissions scenario (RCP2.6) would be very challenging given the current trajectory of carbon dioxide emissions.
How does Australia compare?
By late in this century (2090), Australia’s average warming is projected to be 0.6 to 1.7C for a low emission scenario, or 2.8 to 5.1C under a high emission scenario.
The warming under the high scenario is similar to the global average warming of 2.6 to 4.8C under the high emission scenario reported by the IPCC AR5. However, inland areas of Australia will warm faster than coastal areas.
The new projections should be viewed in the context of what has already been observed. Australia has become 0.9C warmer since 1910. Rainfall has increased in northern Australia since the 1970s and decreased in south-east and south-west Australia.
More of Australia’s rain has come from heavy falls and there has been more extreme fire weather in southern and eastern Australia since the 1970s. Sea levels have risen by approximately 20 cm since 1900.
In future, Australia’s average temperature will increase and we will experience more heat extremes and fewer cold extremes. Winter and spring rainfall in southern Australia is projected to decline while changes in other regions are uncertain.
For the rest of Australia, natural climate variability will predominate over rainfall trends caused by increasing greenhouse gases until 2030. By 2090, a winter rainfall decrease is expected in eastern Australia, but a winter rainfall increase is expected in Tasmania.
Historical climate data can be used as an analogue for the future. The analogue could be a location that currently has a climate similar to that expected in another region in the future.
For example, for a warming of 1.5-3.0C and a rainfall decrease of 5-15%, Melbourne’s climate becomes similar to that of Clare in South Australia, Sydney becomes more like Brisbane, and Brisbane becomes more like Bundaberg in inland Queensland.
Extreme rainfall events that lead to flooding are likely to become more intense. The number of tropical cyclones is projected to decrease but they may be more intense and possibly reach further south. Southern and eastern Australia is projected to experience harsher fire weather. The time in drought will increase over southern Australia, with a greater frequency of severe droughts.
A projected increase in evaporation rates will contribute to a reduction in soil moisture across Australia. There will be a decrease in snowfall, an increase in snowmelt, and therefore reduced snow cover.
Sea levels will continue to rise throughout the 21st century and beyond. Oceans around Australia will warm and become more acidic.
What will Australia look like?
Freshwater resources are projected to decline in far south-west and far south-east mainland Australia. Rising sea levels and increasing heavy rainfall are projected to increase erosion and inundation, with consequent damage to many low-lying ecosystems, infrastructure and housing.
Increasing heat waves will increase risks to human health. Rainfall changes and rising temperatures will shift agricultural production zones. Many native species will suffer from reduced habitats and some may face local or even global extinction.
The most vulnerable regions/sectors are coral reefs, increased frequency and intensity of flood damage to infrastructure and settlements, and increasing risks to coastal infrastructure and low-lying ecosystems.
While reductions in global greenhouse gas emissions would increase the chance of slowing climate change, adaptation is also required because some warming and associated climate changes are unavoidable.
By Pep Canadell, CSIRO
Through burning fossil fuels, humans are rapidly driving up levels of carbon dioxide in the atmosphere, which in turn is raising global temperatures.
But not all the CO2 released from burning coal, oil and gas stays in the air. Currently, about 25% of the carbon emissions produced by human activity are absorbed by plants, and another similar amount ends up in the ocean.
To know how much more fossils fuels we can burn while avoiding dangerous levels of climate change, we need to know how these “carbon sinks” might change in the future. A new study led by Dr. Sun and colleagues published today in PNAS shows the land could take up slightly more carbon than we thought.
But it doesn’t change in any significant way how quickly we must decrease carbon emissions to avoid dangerous climate change.
Models overestimate CO2
The new study estimates that over the past 110 years some climate models over-predicted the amount of CO2 that remains in the atmosphere, by about 16%.
Models are not designed to tell us what the atmosphere is doing: that’s what observations are for, and they tell us that CO2 concentrations in the atmosphere are currently over 396 parts per million, or about 118 parts per million over pre-industrial times. These atmospheric observations are in fact the most accurate measurements of the carbon cycle.
But models, which are used to understand the causes of change and explore the future, often don’t match perfectly the observations. In this new study, the authors may have come up with a reason that explains why some models overestimate CO2 in the atmosphere.
Looking to the leaves
Plants absorb carbon dioxide from the air, combine it with water and light, and make carbohydrates — the process known as photosynthesis.
It is well established that as CO2 in the atmosphere increases, the rate of photosynthesis increases. This is known as the CO2 fertilisation effect.
But the new study shows that models may not have quite right the way they simulate photosynthesis. The reasons comes down to how CO2 moves around inside a plant’s leaf.
Models use the CO2 concentration inside a plant’s leaf cells, in the so called sub-stomatal cavity, to drive the sensitivity of photosynthesis to increasing amounts of CO2. But this isn’t quite correct.
The new study shows that CO2 concentrations are actually lower inside a plant’s chloroplasts — the tiny chambers of a plant cell where photosynthesis actually happens. This is because the CO2 has to go through an extra series of membranes to get into the chloroplasts.
This means that photosynthesis takes place at lower CO2 than models assume. But counterintuitively, because photosynthesis is more responsive to increasing levels of CO2 at lower concentrations, plants are removing more CO2 in response to increasing emissions than models show.
Photosynthesis increases as CO2 concentrations increase but only up until a point. At some point more CO2 has no effect on photosynthesis, which stays the same. It becomes saturated.
But if concentrations inside a leaf are lower, this saturation point is delayed, and growth in photosynthesis is higher, which means more CO2 is absorbed by the plant.
The new study shows that when accounting for the issue of CO2 diffusivity in the leaf, the 16% difference between modelled CO2 in the atmosphere and the real observations disappear.
It is a great, neat piece of science, which connects the intricacies of leaf level structure to the functioning of the Earth system. We will need to reexamen they way we model photosynthesis in climate models and whether a better way exists in light of the new findings.
Does this change how much CO2 the land absorbs?
This study suggests that some climate models models under-simulate how much carbon is stored by plants, and in consequence over-simulate how much carbon goes into the atmosphere. The land sink might be a little bigger — although we don’t know yet how much bigger.
If the land sink does a better job, it means that for a given climate stabilisation, we would have to do a little bit less carbon mitigation.
But photosynthesis is a long, long way before a true carbon sink is created, one that actually stores carbon for a long time.
About 50% of all CO2 taken in by photosynthesis goes back to the atmosphere soon after through plant respiration.
Of what remains, more than 90% also returns back to the atmosphere through microbial decomposition in the soils and disturbances such as fire over the following months to years — what stays, is the land sink.
Good news, but not time for complacency
The study is a rare and welcome piece of possible good news, but it needs to be placed in context.
The land sink has very large uncertainties, they have been well quantified, and the reasons are multiple.
Some models suggest that the land will continue to absorb more carbon all throughout this century, some predict it will absorb more carbon up to a point, and some predict that the land will start releasing carbon — becoming a source, not a sink.
The reasons are multiple and include limited information on how the thawing of permafrost will effect large carbon reservoirs, how the lack of nutrients could limit the further expansion of the land sink, and how fire regimes might change under a warmer world.
These uncertainties put together are many times bigger than the possible effect of the leaf CO2 diffusion. The bottom line is that humans continue to be in full control of what’s happening to the climate system over the coming centuries, and what we do with greenhouse emissions will largely determine its trajectory.
Pep Canadell receives funding from the Australian Climate Change Science Program.