Preventing the worst impacts of climate change will require zero carbon emissions AND removal of carbon from the atmosphere

In 2015 the heads of 195 countries signed the Paris Climate Accord, which set the goal of keeping global average temperatures from rising “well below 2°C," and to make every effort to keep the temperature from rising more than 1.5°C. Experts estimate keeping warming to 1.5°C will save $30 trillion in costs compared to a 2°C temperature rise.

Scientists now understand that rises in global temperature are a response to the amount of carbon dioxide and other greenhouse gases (e.g., methane, nitrous oxide) that have accumulated in the atmosphere. Therefore, we can estimate our "carbon dioxide budget" - the amount of carbon dioxide we can emit in order to stay within, say, 1.5 degrees of warming. Dr. Richard Millar of Oxford estimates that limiting emissions to 880 Gt from 2015 onwards (equivalent to about 20 years of current annual emissions) will give us a 2/3 chance of staying within 1.5 degrees Celsius.

In almost all scenarios modeled by the Intergovernmental Panel on Climate Change (IPCC), CO2 emissions will need to be reduced to net zero globally around 2050 and in most cases fall below zero afterwards. This means that in almost every possible scenario that limits warming to 1.5 degrees, drastic emissions cuts AND negative emissions are required.

The chart below shows the ways in which humans release carbon dioxide into the atmosphere. Every year, humans release about 39 Gigatons (Gt) of CO2; 13 Gt from energy, 2 Gt from buildings, 7 Gt from industry, 7 Gt from transport, 4 Gt from agriculture, and 5 Gt from land use change.

Luckily, nature has a way of helping mitigate emissions; each year 12.5 Gt are captured in land (e.g., through soil and trees), and 9.5 Gt is captured by the ocean. On net, each year we emit 17 Gt of CO2 (39 Gt minus 22 Gt).

It's important to recognize that we have the equivalent of 3,100 Gt CO2 of fossil fuel reserves, so if we burn a significant portion of our fossil reserves we will have absolutely no hope of staying within 1.5 degrees Celsius.

Similarly, 2,000 Gt of CO2 is trapped in vegetation (e.g., trees and plants) and 5,500 Gt is in soils. When trees are cut down and peatlands are drained, the stored carbon is released into the atmosphere. Similarly, when soil is disturbed on farms through tillage or through overuse of chemicals such as fertilizer, carbon is released into the atmosphere. Therefore, it's crucial to prevent these large stores of carbon - from vegetation and soil - from being released into the atmosphere. "Climate tipping points" are situations when higher temperatures lead to feedback loops that emit increasing amounts of carbon into the atmosphere. Some examples:

• Melting permafrost leads to significant release of methane gas, which in turn leads to higher temperatures
• Deforestation in the Amazon leads to reduced rainfall, which then results in massive "dieback" of trees, releasing carbon into atmosphere
• Rising temperatures leads to increased wildfires, which release more carbon

We can remove carbon from the atmosphere in two ways: through engineering, or through nature. Engineered solutions include technical solutions such as direct air capture and enhanced oil recovery. Natural solutions include responsible forestry and afforestation, regenerative farming, and biochar.

One focus of my climate research is in natural solutions, which can be scaled to large volumes, and also can often be achieved highly cost-effectively. Natural solutions, in combination with dramatic emissions reductions, will be an important part of preventing a runaway rise in temperatures.

However, it is important to note that natural climate solutions can be reversed, either by human actions or by the "tipping points" described above. Wildfires, logging, tree decay, and tillage can all cause stored carbon to be released back into the atmosphere.

What we do in the next decade will be critical in determining whether we can stay on a path to 1.5 degrees of warming or less. The level of carbon reduction needed is drastic, so governments and corporations will need to take meaningful action; the priority must be immediate emission cut, but we also do need R&D for solutions that will help permanently remove carbon from the atmosphere.