Transcriber: Victoria Peredo Acevedo Reviewer: Amanda Zhu
As we walked over the dry grass and prickly shrubs,
I would occasionally reach down
and grab a handful of thyme or rosemary leaves,
bring them close to my nose and breathe in
and feel a sudden hunger for roasted potatoes.
The aromatic atmosphere
of Mediterranean pine and oak forests in northeastern Spain
were unfamiliar to me.
I’m used to the much wetter maple forests
of Eastern Canada.
But my Ph.D. fieldwork took me
to the Penedès wine producing region of Catalonia,
where upland forests, like the ones we were exploring,
wisely watch over the drunkenly curling grape vines and stubborn olive trees
in the fields below.
It is a picture-perfect rural landscape,
with the Sun, the soil, and the plants all in a secret agreement to never change,
the land seemingly frozen in time, producing the same crops for centuries.
However, deep within that forest, amongst those wonderfully smelling herbs,
there is another more revealing story being told,
a story that directly challenges humanity’s proud claim
to be the eternal masters of our agricultural landscapes.
Not far from the active vineyards and olive groves,
my colleagues and I found exactly what we came for:
proof that this forest was once under total cultivation.
They were ancient stone walls once used for agricultural terraces,
dipping and peeking through the dry soil and tree roots.
There was an abandoned stone hut
where a farmer would long ago store supplies
or take a much needed break from the Sun and have a siesta.
But even without all this structural evidence,
the fact that there were grapevines and olive trees
growing suspiciously in a forest normally dominated by much different species
was proof enough.
All these things betrayed the charade the forest was performing.
This land was once used for agriculture,
just like all the actively cultivated fields only a few hundred meters away.
We were standing in just one example of what can happen
after agricultural land abandonment.
This is a land use change that has had a remarkable impact,
not only on Mediterranean landscapes after decades of rural depopulation,
but in all agricultural regions of the world throughout history
for many different reasons.
Although it’s quite a tricky process to map and quantify
because it’s often underreported,
some estimates suggest
over 400 million hectares of agricultural land
has been abandoned since the year 1700,
with five percent of global croplands abandoned since 1992.
Abandonment implies a cessation of agricultural practices
and in many cases, the regeneration of pre-agricultural ecosystems.
So this means the return of our planet's vital and naturally occurring grasslands,
shrublands and forests,
the recovery of depleted and degraded soils,
and even the return of some animals not seen for generations.
But in light of the intensifying challenges posed by climate change,
arguably the most important feature of these post-agricultural landscapes
is that they can also be powerful natural carbon sinks.
You see, not only has modern agriculture
become one of the largest contributors of greenhouse gases
since the advent of intensive practices
but nearly all types of agriculture have long been draining our soils of carbon.
In fact, over the past 12,000 years,
agriculture has taken over 100 billion tonnes of carbon from our soils.
Post agricultural landscapes, on the other hand,
can reverse some of this damaging legacy by recapturing some of that lost carbon
and holding onto it for long enough
to help diminish the greenhouse gas effect
that is now warming our planet far too rapidly.
This promising nature-based solution to combat climate change
is all because of a process called “soil carbon sequestration,”
and some of the highest sequestration rates
observed on unmodified soils
have been on agricultural soils that were taken out of production
and allowed to regenerate naturally or through assisted restoration.
So soil carbon sequestration
is simply the removal of atmospheric carbon dioxide
by plants
and the storage of this carbon in soils.
Now, although it’s not a permanent transfer of carbon,
it can happen
over a long enough time periods and across large enough spatial scales
to make a real difference in the global carbon budget.
Soils, after all, already hold more carbon
than the entire atmosphere and all living biomass combined.
And following the cessation of agricultural practices,
and if the soil is not degraded beyond recovery,
it will spontaneously begin rebuilding the conditions for carbon accumulation.
So, now that the soil structure is not being damaged by tillaging,
the pores in the aggregates that help protect and stabilize carbon
begin to redevelop.
And while this is happening,
the newly sprouting plants
begin drawing down carbon dioxide from the atmosphere
and pumping this carbon down through their roots
to the soil microbes below in exchange for nutrients,
further building the soil carbon stock.
And what’s important to remember
is that these plants, if they’re never removed from the land,
like crops and even crop residues are often removed from crop lands,
they live, they die, and they remain on the soil.
They’re carbon-based biomass
left to be decomposed and transformed into carbon-rich soil organic matter
by all the living organisms populating soils,
from the very small bacteria and fungi to the very large earthworms.
So while it’s really important to remember that there’s many exciting ways,
and necessary ways,
to promote soil carbon sequestration on active agricultural landscapes,
you could say
that the simplest and most efficient management decision to make
to build back soil carbon stocks
is to stop farming.
And indeed,
when I took those soils from that forest back to the lab and analyzed them,
the increase in carbon over time
since the cessation of agricultural practices
was clear.
So I started to wonder,
how much could we rely on a process like abandonment
to draw down more atmospheric carbon across larger spatial scales,
scales that could make a real difference in the global carbon budget?
As it turns out,
one of the largest ever human induced land carbon sinks
was a result of the 45 million hectares of crop lands that were abandoned,
starting in 1990,
following the collapse of the Soviet Union.
And these vast expanses of post-agricultural landscapes
were recolonized by forests and re-saturated
with over 40 million tonnes of carbon per year.
And it’s examples like this
that have helped fuel ongoing, ambitious restoration programs,
like the Conservation Reserve Program at the United States
or the Grain for Green program of China,
where strategically selected agricultural land
is intentionally abandoned to jumpstart ecosystem recovery.
But despite the success of programs like these
and the clear climate change mitigation potential of post-agricultural landscapes,
there are still many uncertainties and questions and valid counter arguments
that we must address as we enter the UN Decade [on] Ecosystem Restoration
this decade.
And as we strive to achieve a global level
of sustainable land management as soon as possible.
How do we even know that the many costs of taking land out of production
would ever be outweighed by the carbon sink benefits?
We may say that this is a great nature-based solution
to combat climate change,
but that’s only if we take people out of the equation.
Aside from the glaringly obvious issue
of maintaining and ensuring global food security,
there are many other social, economic and cultural dimensions
to a proposal like this
that cannot be simply swept aside in the name of a climate crusade.
Back in the Mediterranean, for example,
with my field sites,
abandonment as a concept
is commonly interpreted as as the end of traditional rural landscapes
and the economic depression of rural societies.
And ironically, abandonment can even have negative ecological impacts,
like a loss in biodiversity,
compared to the vibrant agroecosystems that were there before,
or an increased risk in wildfires or even a loss of soil due to erosion
if the vegetation recovery is stalled.
And similarly, planting trees on crop lands
is no longer the clear one-size-fits-all restoration answer.
Recent findings have taught us once again
to work with nature rather than force nature
and to consider local land rights at the start of the discussion.
Whether agricultural lands are unintentionally abandoned
or intentionally converted back to natural vegetation,
our ability to accurately account for their role
in climate change mitigation
is currently hindered by limited data.
Because soils are so incredibly diverse,
they respond to land use change in so many different ways.
We urgently need to know
how all the different types of agricultural land
that we find around the world
would respond to something like abandonment,
considering all the factors that we know play a role in carbon accumulation,
like the soil properties, the local climate,
and the agricultural management history.
And most importantly,
we need to better constrain
the temporal aspects of soil carbon sequestration
on these landscapes,
asking questions like “How long will the carbon accumulate for?
When will it reach a saturation point?
And how long will it stay in the soil?” -
essentially asking, How much can we rely on a process like this?
Fortunately, by combining available field data
with new approaches in modeling and remote sensing,
we are on our way
to a better quantification of our shared land resources
and their soil carbon sequestration potentials.
So agricultural land abandonment
is just one important piece of this data puzzle
that my colleagues and I are now addressing at the global scale.
But to be ready to use this kind of carbon sink data,
I also think we need to start shifting our focus
to some of those difficult follow-up questions,
like where and when should we start
to strategically abandon agricultural lands
and how much land should we abandon for carbon sink benefits?
Our soils have given us so much throughout history,
and through this process, they become degraded, depleted, and tired.
And I suggest that besides using the term “abandonment,”
it’s time to change that term.
It’s time to use a term that’s more realistic,
like “retirement.”
And in doing so,
we can put a lot of carbon back where it belongs.
Thank you.