The #IPCC special report on the land sector is out.

It's the culmination of the hard work of 107 experts from 52 countries.

I've got some big takeaways. Buckle in – it's going to be a long thread.

Some quick background:

Land and ocean warm at different rates (thank you @RARohde) for the great figure.
Land is both a carbon source and sink. The balance between the strength of sources and sinks determines overall impact of land on GHG emissions. Humans already use a LOT of land (almost ¾ of all ice-free terrestrial surface, according to the IPCC graphic below).
About 22% of total anthropogenic GHGs (expressed as #CO2 equivalent) come from #agriculture, forestry, and other land use (or AFOLU, if you’re fancy). Half comes from CO2 emissions (deforestation), and the rest is methane and nitrous oxide.
Tropical #deforestation has resulted in major CO2 losses from 2007-16 WRI estimates there has been an uptick of tropical deforestation (63% higher) than even 10 years ago. But so far, the uptake is keeping pace (and even outpacing) the CO2 losses.
Beyond CO2, ag is a big emitter of CH4 (livestock burps = 1/2 of global CH4 emissions) and N2O (3/4 of global emissions) from N fertilizer. The use of inorganic synthetic N has increased almost 9-fold globally and 70% of global fresh water is used to irrigate cropland.
Soil loss from conventionally tilled land is exceeding the rate of soil formation by >2 orders of magnitude.

We have 12,000 years of soil debt to repay.

“What we do to our soils we do to our climate - and ourselves”…
The opening to Chapter 1 of the full report does not mince words:

“Neither our individual or societal identities, nor the world’s economy would exist without the multiple resources, services and livelihood systems provided by land ecosystems and biodiversity.”
So that’s where we are.

Now a couple of BIG takeaways from this latest IPCC report.

First, #ClimateChange and land degradation are THREAT MULTIPLIERS for people already living precariously, populations already sensitive to extreme weather, food insecurity, poverty.
Second, from 2008-2017, ~30% of total anthropogenic CO2 emissions globally or ~6.2 Gt CO2/yr was “removed”.

For scale, one car in the US emits 4.6 metric tons of CO2/yr & 1 Gt of CO2 is equivalent to emissions from 232 million cars (and please chime in if my math is wrong).
In other words, the land sink is meaningful.

You might be wondering - how did we end up with a land sink, and isn’t that a good thing?
Over the last 3 decades, the planet has generally greened in response to lots of irrigation (increased corn and soy production), some forest conservation efforts, CO2 fertilization, longer growing seasons, and N deposition (and therefore fertilization).
But greening is not happening uniformly everywhere. Some places (high latitudes) are greening while other places (southwestern US) are browning.

Plants, especially C3 plants, do well with extra CO2 (we call this CO2 fertilization).
Extra CO2 improves plant water use efficiency and boosts productivity, especially in drylands.

But there’s a limit: The CO2 fertilization effect is only possible with enough nutrients.…
The balance between greening and browning in part determines the amount of terrestrial carbon uptake (more green, more uptake).

But as GHG emissions go up, the land sink is already having a hard time keeping up.
The shift in land cover also affects something called surface albedo, a measure of the amount of sunlight that gets reflected back into space.

Like a dark shirt on a sunny day, the more sunlight that gets absorbed, the warmer the Earth’s atmosphere.
Predicting how surface albedo will change over time or the feedbacks it will have with warming is challenging, but we’re confident it will have an impact.

(Medium confident, if you want to get technical – see the screenshot for more.)
At the same time, the four pillars of food security – availability, access, utilization, and stability – are already affected by climate change, especially increased frequency of extreme weather events.

And changes in land cover will affect agricultural systems.

Longer growing seasons and more CO2 may help high latitude crops, but low latitude crops are already losing yields.

Animals are also impacted w/lower growth rates and fertility declines, more pests and disease.
So the question is, how do we keep feeding a growing population, using less land, less water, less synthetic fertilizer, and removing as much CO2 from the atmosphere as possible?
Answering that question will require us to harness the full power of the land carbon sink and grappling with some trade-offs.

There are many co-benefits of changing how we produce food and manage forests to mitigate climate change.

But there is always a cost.
And our opportunity window is rapidly closing, according to this IPCC report.

“Confidence is very high that the window of opportunity – the period when significant change can be made, for limiting climate change within tolerable boundaries – is rapidly narrowing.”
No time to waste, harnessing the full power of carbon removal will take ingenuity, collaboration, and some vision!

/End Thread
PS - the report takes a deep dive into land degradation, which I will leave for another thread.

For an epic IPCC report summary, check out @CarbonBrief…
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