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In this model, almost all carbon removal occurs through a process known as bioenergy with carbon capture and storage, or BECCS. Essentially, this requires growing crops that absorb CO2 and then using the harvested biomass to produce heat, electricity or fuel, capturing and storing any resulting emissions. But despite the billions and billions of tons of carbon removal that climate models rely on through the BECCS, this has only been done in small projects to date.

Other technical approaches are also immature, including machines to suck carbon and various ways to accelerate the natural processes by which minerals and oceans absorb and store CO2. It has proven difficult to design systems to reliably stimulate and measure carbon removal using natural systems such as forests and soil.

The IPCC assessment on Monday noted that there are many other constraints and challenges.

First, while carbon removal does reduce the level of greenhouse gases in the atmosphere, the report notes that this effect can be offset to some extent. Modeling studies have shown that the oceans and land are beginning to emit more CO2 in response to changes in atmospheric chemistry over time periods, offsetting the benefits.

In addition, while carbon removal can gradually mitigate rising temperatures and ocean acidification, it does not magically reverse all climatic influences. Remarkably, it will take centuries for the oceans to return to the level at which we built coastal cities, the report says. Ice sheets, coral reefs, rainforests and some species can suffer near-irreversible damage, depending on how much warmer the world gets before the world cuts emissions and increases carbon removal.

The fifth chapter of the report lays out many other trade-offs and unknowns associated with virtually all potential approaches to large-scale carbon removal.

Carbon suction machines require a lot of energy and materials. Planting more trees for carbon sequestration or crops for fuel will compete with growing food for the world’s growing population.

Climate change alone will undermine the ability of forests to absorb and store carbon dioxide, as the risks of droughts, wildfires and insect infestations increase with increasing temperatures. And there is still considerable scientific uncertainty regarding the side effects of various ocean-based approaches on marine ecosystems.

The good news is that there are many ways to remove carbon from the air, and an increasing number of research groups and companies are working to develop more efficient and cheaper methods. But, as is clear from Monday’s report, we are lagging far behind in the very high stakes race.