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 CO 2 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 CO 2 per year to about 100 billion tonnes of CO 2 in 2100, when atmospheric CO 2 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 CO 2 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 CO 2 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 CO 2 , 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 CO 2 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 CO 2 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 CO 2 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 CO 2 each year in the most optimistic scenario, to between 25-40 billion tonnes CO 2 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 CO 2 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 CO 2 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 CO 2 levels in the atmosphere.

No CDR or suite of CDR technologies exists capable of removing the levels of CO 2 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 CO 2 , 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.