深度解析：非永續土地利用與氣候變遷密切相關

This morning in Geneva, the Intergovernmental Panel on Climate Change (IPCC) published its special report on climate change and land.

The land provides the "food, feed, fibre, fuel and freshwater" without which human society and its economy "could not exist", the report says. This provision is under threat from rising global temperatures and "unprecedented" rates of land and freshwater exploitation in recent decades, the report warns.

The full title of the report gives an indication of the catalogue of interlinked and overlapping issues it covers, including "climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems".

In this detailed Q&A, Carbon Brief unpacks what the report says about how climate change affects the land and vice versa, as well as other key topics such as food security, negative emissions and how to tackle the overlapping challenges associated with how humans use the land.

Why did the IPCC produce this report?

The IPCC's land report was published at the end of a week-long approval plenary held in Geneva, Switzerland. At this session, government delegates approved the 43-page "summary for policymakers" (SPM) overview report line-by-line, culminating in a marathon overnight session. Once the SPM was agreed, the IPCC also released the full technical report that runs to nearly 1,400 pages, plus supplementary materials.

(Following the approval of the SPM, there are some updates that need to be made to the full report to ensure it is consistent with the revised SPM. These have not yet been made and so the individual chapters are subject to changes listed in a "trickle-back" document.)

The land report was originally commissioned by the IPCC at an April 2016 meeting in Nairobi (pdf). A draft outline for the report was subsequently adopted at a meeting in Guadalajara, Mexico (pdf), in March 2017.

The author team (pdf) of coordinating lead authors, lead authors and review editors– selected in July 2017 – is made up of 107 experts from 52 countries. They have been supported by dozens of contributing authors on each chapter.

Tomorrow the @IPCC_CH will release the #SRCCL—the Special Report on #ClimateChange and Land.

The report brings together the most knowledgeable experts and the most reliable science to explore the interlinkages between climate change and land usage.pic.twitter.com/LrIvmeo7AW

— IPBES (@IPBES) 2019年8月7日

Speaking at a press conference this morning, IPCC chair Hoesung Lee explained what he viewed to be the "high level findings" of this report:

"Land is where we live. Land is under growing human pressure and is part of the solution, but land can not do it all."

The land report is the second special report that the IPCC is due to publish this year. The second – on oceans and the cryosphere– is due in September this year. The IPCC also published a special report on 1.5C of warming in October 2018.

The aim of these special reports is to provide "an assessment on a specific issue". They complement the main "assessment reports" that the IPCC publishes every five or six years.

The new report is the first from the IPCC with land as a central focus, since the special report on land use, land-use change and forestry in 2000.

Chapter four of the full report sets out what is included in the definition of "land":

"The terrestrial portion of the biosphere that comprises the natural resources (soil, near surface air, vegetation and other biota, and water), the ecological processes, topography, and human settlements and infrastructure that operate within that system."

And chapter one opens by emphasising the importance of this land to human civilisation:

"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."

The report says these contribute an annual value of $75-85tn to the global economy (in the year 2011, based on the value of the US dollar in 2007), putting it far in excess of the world's combined GDP.

As the figure below – from the SPM – illustrates, humans currently appropriate close to three-quarters of the ice-free land globally. Around 12-14% of this area has been converted for growing crops (yellow); 22% is made up of managed or planted forests (blue); and 37% is grassland adopted for grazing and other uses (green). The rectangles in orange on the right-hand side indicate the land currently unused by humans.

The geographic spread of land use and the "large appropriation of multiple ecosystem services and the loss of biodiversity" are "unprecedented in human history", the report says. Human use – at varying intensities – "affects about 60-85% of forests and 70-90% of other natural ecosystems (e.g., savannahs, natural grasslands)". And land use has caused an 11-14% drop in global biodiversity, the report notes.

Not only is human use of the land more pervasive than ever, it is set against a background of a warming climate. The report says:

This warming – along with associated changes in rainfall patterns – has "altered the start and end of growing seasons, contributed to regional crop yield reductions, reduced freshwater availability, and put biodiversity under further stress and increased tree mortality", the report notes.

In other words: climate change is magnifying the pressures that humans are already putting on the land. But climate change is itself in part a result of the way in which humans use land, the report explains:

"Conversion of natural land, and land management, are significant net contributors to GHG [greenhouse gas] emissions and climate change, but land ecosystems are also a GHG sink."

"It is not surprising, therefore" the report notes, that the land plays a prominent role in many of the national commitments to cut emissions that have been pledged under the Paris Agreement.

But time to tackle rising global temperatures is short, the report warns:

"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."

Because of the close links between unsustainable land use and climate change, solving one is made more difficult by the other, the report says:

"Enhancing food security and reducing malnutrition, whilst also halting and reversing desertification and land degradation, are fundamental societal challenges that are increasingly aggravated by the need to both adapt to and mitigate climate change impacts without compromising the non-material benefits of land."

This also means that tackling one can have co-benefits – and trade-offs – for the other. This is perhaps the crux of the motivation for the report. As the SPM points out:

"Many land-related responses that contribute to climate change adaptation and mitigation can also combat desertification and land degradation and enhance food security."

But "none of these response options are mutually exclusive", chapter one says, which means it is working out how to combine them – in a "context-specific manner" – that is most likely to "achieve co-benefits between climate change mitigation, adaptation and other environmental challenges in a cost- effective way".

What is land degradation?

Chapter four of the new IPCC report defines land degradation as:

"A negative trend in land condition, caused by direct or indirect human-induced processes including anthropogenic climate change, expressed as long-term reduction or loss of at least one of the following: biological productivity, ecological integrity, or value to humans."

This definition encompasses temporary or permanent decline in quality of soil, vegetation, water resources or wildlife – or the deterioration of the economic productivity of the land, such as the ability to farm the land for commercial or subsistence purposes.

Table 4.1 – which runs to five pages – details the major degradation processes and their connections with climate change. It includes, for example, different types of erosion – the gradual breaking down and removal of rock and soil. This is typically through some force of nature, such as wind, rain and/or waves, but can be exacerbated by activities including ploughing, grazing or deforestation.

A loss of soil fertility is another form of degradation. This can be through a loss of nutrients, such as nitrogen, phosphorus and potassium, or a decline in the amount of organic matter in the soil. Other processes include: a loss or shift in vegetation type and cover, the compaction and hardening of the soil, an increase in wildfires, and a declining water table through excessive extraction of groundwater.

(This definition solely focuses on degradation driven – either directly or indirectly – by humans, it doesn't include natural processes such as volcanic eruptions and tsunamis.)

The report also specifies that forest degradation is land degradation in areas that are still predominantly forest, while "deforestation refers to the conversion of forest to non-forest that involves a loss of tree cover and a change in land use".

The total amount of forest across the world declined by around 3% from 1990-2015, according to multiple lines of evidence, the reports says.

However, the change in the rate of deforestation in recent years differs between estimates, it adds, with satellite data suggesting that forest loss is accelerating and on-the-ground estimates suggesting that it is slowing down.

There is also little clarity surrounding the extent of land degradation in forests, says chapter four of the report. "The lack of a consistent definition of forest degradation also affects the ability to establish estimates of the rates or impacts of forest degradation. Moreover, the [scientific] literature at times confounds [mixes up] estimates of forest degradation and deforestation."

"The entire spectrum of factors" that drive land degradation range from "very short and intensive events such as individual rain storms of 10 minutes removing topsoil or initiating a gully or a landslide to century scale slow depletion of nutrients or loss of soil particles", the report says.

The long-term nature of some impacts is reflected in the fact that land degradation has "accompanied humanity at least since the widespread adoption of agriculture during Neolithic time, some 10,000 to 7,500 years ago, and the associated population increase", the report notes.

But, of course, not all human impacts on land result in degradation, the report stresses:

"There are many examples of long-term sustainably managed land around the world (such as terraced agricultural systems and sustainably managed forests)…We also acknowledge that human use of land and ecosystems provides essential goods and services for society."

How does climate change affect land degradation?

"Land degradation is inextricably linked to several climate variables, such as temperature, precipitation, wind, and seasonality," the report says. This means that there are many ways in which climate change and land degradation are related, it adds:

"The linkages are better described as a web of causality than a set of cause-effect relationships."

Climate change can affect the land through both gradual changes in temperature and rainfall patterns, as well as changes in the "distribution and intensity of extreme events", the report notes.

There are cases where, even when climate change exerts a direct pressure on degradation processes, "it can be a secondary driver subordinated to other overwhelming human pressures", the report says. But it identifies three processes in which climate change is the "dominant global or regional pressure".

These are: "coastal erosion as affected by sea level rise and increased storm frequency/intensity, permafrost thawing responding to warming, and increased burning [of wildfires] responding to warming and altered precipitation regimes".

For example, the report has "very high confidence" that erosion in coastal areas as a result of sea level rise will increase worldwide. It adds:

"In cyclone prone areas (such as the Caribbean, Southeast Asia, and the Bay of Bengal) the combination of sea level rise and more intense cyclones, and in some areas also land subsidence, will pose a serious risk to people and livelihoods, in some cases even exceeding limits to adaption,"

Climate change "not only exacerbates many of the well acknowledged ongoing land degradation processes" of managed landscapes, such as croplands and pasture, but it "becomes a dominant pressure that introduces novel degradation pathways in natural and semi-natural ecosystems", the report says.

Warming conditions and changing rainfall patterns will also "trigger changes in land- and crop management, such as changes in planting and harvest dates, type of crops, and type of cultivars", the report notes, "which may alter the conditions for soil erosion".

The report also notes that "changes in extreme weather and climate have negative impacts on food security through regional reductions of crop yields". For example, it says that on average over recent decades, around 10% of cereal production has been lost globally because of extreme weather events.

And climate change is already influencing "species invasions and the degradation that they cause by enhancing the transport, colonisation, establishment, and ecological impact of the invasive species", the report adds.

When rainfall patterns change, it is expected to drive changes in vegetation cover and composition, "which may be a cause of land degradation in and of itself", the report says, adding: "vegetation cover, for example is a key factor in determining soil loss through both water and wind erosion".

The air can generally hold around 7% more moisture for every 1C of temperature rise. This means a warmer climate has the potential for more intense rainfall events, which "increase the erosive power of rainfall (erosivity) and hence increase the likelihood of water erosion", the report says.

For example, in central India,"there has been a threefold increase in widespread extreme rain events during 1950-2015, which has influenced several land degradation processes, not least soil erosion".

One potentially lethal consequence of increased rainfall and the accompanying land degradation is an uptick in landslides. However, the report notes that while there is "strong theoretical reason" to associate extreme rainfall with these events, there is currently not a lot of evidence linking climate change and landslides.

Overall, the report has "high confidence" that the "frequency and intensity of some extreme weather and climate events have increased as a consequence of global warming and will continue to increase under medium and high emission scenarios".

A combination of heat and drought threatens already drought-prone areas, the report warns:

"Heatwaves are projected to increase in frequency, intensity and duration in most parts of the world and drought frequency and intensity is projected to increase in some regions that are already drought prone, predominantly in the Mediterranean, central Europe, the southern Amazon and southern Africa."

Extreme heat events can reduce photosynthesis in trees, restrict growth rates of leaves and reduce growth of the whole tree, the report notes. Forests can become less resilient to future heat stress as extreme events occur more often, the report adds, and "widespread regional tree mortality may be triggered directly by drought and heat stress (including warm winters) and exacerbated by insect outbreak and fire".

On a related note, climate change is "playing an increasing role in determining wildfire regimes alongside human activity", the report says, "with future climate variability expected to enhance the risk and severity of wildfires in many biomes such as tropical rainforests".

Under medium and high scenarios of future GHG emissions, "global warming will exacerbate heat stress thereby amplifying deficits in soil moisture", the report says. This will "increase the rate of drying", the report adds, "causing drought to set in quicker, become more intense and widespread, last longer and could result in an increased global aridity".

Changes to the planet's vegetation cover due to climate change are likely to have far-reaching consequences, according to the report. According to one study, if temperatures rise by 2C above pre-industrial levels, two thirds of key biodiversity areas are projected to remain intact, whereas only a third will at 4.5C. In regions where plants decline, land degradation is expected to occur as vegetation provides a vital safeguard against erosion.

The chart below, from the SPM of the report, illustrates how the level of risk to different aspects of the land surface varies with rising temperature. For example, the risk to water scarcity in drylands (left-most bar) increases from "moderate" (yellow shading) in the present climate to "high" (red) in a 2C warmer climate and "very high" (purple) with 3C of warming.

The chart shows that the risks to permafrost (third bar from the right) and food supply stability (right-most bar) are expected to increase particularly rapidly with rising temperatures.

The report does, however, stress that some aspects of climate change can improve the condition of the land. One example is "CO2 fertilisation", where higher levels of CO2 in the atmosphere bolsters plant growth, the report says:

"Increasing CO2 levels in the atmosphere is a driver of land improvement even if the net effect is modulated by other factors, such as the availability of nitrogen and water."

However, as a modelling study of the Amazon published this week indicates, the benefits for tree growth could be restricted where soil nutrients are limited. Other benefits could include longer growing seasons in high latitudes thanks to warmer shoulder seasons of spring and autumn – assuming they are not affected by summer droughts – and increasing levels of atmospheric CO2 improving how efficiently trees can use water.

Despite some benefits to land as the climate changes, the report warns that "negative impacts dominate", adding that these have already been documented and "are predicted to increase".

The report also looks at the evidence for links between land degradation and human-caused climate change – a field of research known as "attribution" that is typically applied to extreme weather events.

Proving a definitive climate change-related influence on land degradation is "extremely challenging", the report says. This is because degradation is a "complex phenomenon often affected by multiple factors", related to climate, ecology, land type, and management practices.

"There is not much research on attributing land degradation explicitly to climate change," the report notes, "but there is more on climate change as a threat multiplier for land degradation".

The most important direct links between climate change and land degradation are the result of increasing temperatures, changing rainfall patterns, and intensification of rainfall, the report says.

Scientists have shown that each of those links is being affected by human-cause greenhouse gases. Carbon Brief, for example, has previously explained how humans emissions and activities have caused around 100% of the warming observed since 1950. And the IPCC's special report on 1.5C concluded that human-induced global warming has already caused an increase in the frequency, intensity and/or amount of heavy precipitation events at the global scale. Moreover, "the intensification of the hydrological cycle as a result of human-induced climate change is well established", the land report says.

The report concludes that there is "high confidence" and "robust evidence" that "rainfall changes attributed to human-induced climate change have already intensified drivers of land degradation, but attributing land degradation to climate change is challenging because of the importance of land management."

In addition, the report says: "[S]ince climate change exacerbates most degradation processes it is clear that unless land management is improved, climate change will result in increasing land degradation."

How does the land contribute to climate change?

The land plays a key role in storing greenhouse gases. From 2008-17, the land absorbed 30% of the world's greenhouse gas emissions, according to the report.

The land takes in CO2 from the atmosphere when trees and other types of vegetation carry out photosynthesis – the process needed for plant growth. In this process, plants use CO2 to build new materials such as shoots, roots and leaves. This means that, as long as plants are alive, they can act as long-term "sinks" of CO2. Scientists estimate that up to 45% of carbon stored in the land is held by the world's forests.

Another major way that the land holds carbon is through its soils, which typically gain carbon through plant material, crop residues and animal manure. The top metre of the world's soils is thought to contain three times as much carbon as the entire atmosphere.

The report says that the ability of the land to absorb greenhouse gases is currently being aided by "increasing atmospheric CO2 concentration [and] a prolonged growing season in cool environments". To read more about these issues, please see: "How does climate change affect land degradation?"

Though the land acts as a major store of carbon, it can also be a greenhouse gas emitter.

The leading cause of greenhouse gas release from the land comes from human activity on its surface. The report finds that around 23% of global greenhouse gas emissions released from 2007-16 came from agriculture, deforestation, peatland degradation and other types of land use.

The major driver of CO2 release is deforestation and other types of vegetation loss, according to the report, whereas the major driver of methane and nitrous oxide emissions is agriculture.

The chart below, taken from the SPM, gives a picture of how emissions of all three greenhouse gases have changed since 1961. The chart shows (1) CO2 emissions from forestry and other land-use change, (2) methane emissions from livestock and (3) nitrous oxide emissions from agriculture.

The chart shows that CO2 emissions from land use decreased in the 1960s before stabilising at "high levels" in the 21st century. From 2008 to 2017, CO2 emissions from land-use change totalled around 1.5bn tonnes a year, the report says.

Though land-use emissions are at "high levels", the land is still a net carbon sink, the report says. This means that the land currently takes in more CO2 than it emits.

This is because emissions from human land use are currently being more than offset by land processes that remove CO2 from the atmosphere. An idea of this is given in the chart below, which is taken from page 30 of chapter two.

On the chart, the impact of human activity (agriculture, forestry and other land use, AFOLU) on land emissions is shown in red, while the impact of natural processes and "indirect" human influence is shown in green. Blue shows the overall emissions from land when both factors are considered.

The chart shows that the "net" removal of CO2 by the land totalled an average of 6bn tonnes a year from 2007-2016. (This is shown by the blue bar.)

A further breakdown of human activity emissions (AFOLU; red pluses and minuses) shows that people are currently causing more CO2 emissions than they are able to offset.

The largest source of CO2 losses from 2007-16 was tropical deforestation, the report says. (Analysis from the World Resources Institute found that tropical deforestation in the past three years was 63% higher than in the preceding 14 years.)

Human activity on land does account for some CO2 removal, the chart shows. This is mainly due to reforestation and other types of "greening" including agriculture, the report says.

However, the largest CO2 removals are carried out by "indirect effects" (green pluses and minuses). These are the natural processes, such as those provided by trees and soils, as well as the "indirect" human-driven effects, such as the CO2 fertilisation effect (discussed in more detail above). There is still a large degree of uncertainty surrounding the true extent of these effects, the report notes, which is why question marks are included on the chart.

While CO2 emissions from land use have stabilised at "high levels", methane emissions from agriculture have increased by 1.7 times since 1961, the report finds. Methane is a greenhouse gas that is 32 times more potent than CO2 over a 100-year time period.

The major sources of methane emissions from land are currently "ruminants and the expansion of rice cultivation", according to the SPM.

"Ruminants" are animals that have specialised stomach bacteria capable of digesting tough and fibrous material such as grass. The digestive process causes the animals to belch out methane. The most commonly reared ruminants are beef and dairy cows.

Livestock production is currently responsible for around 33% of global methane emissions, according to chapter two of the report, and for 66% of agricultural methane.

Asia is the region that produces the most methane from livestock production, the report adds. The region accounts for 37% of livestock methane emissions and gases from the region have been growing at about 2% every year since 2000, the report says.

Rice farming, meanwhile, causes methane release when the crop is grown in flooded paddies. In flooded conditions, water can become "anoxic"– depleted of oxygen – which causes the bacteria that break down plant matter to produce methane.

Other smaller sources of land methane emissions include animal manure, waste burning and peatlands in the northern hemisphere. Northern peatlands contribute around 10% of global methane emissions, says page 41 of chapter two:

"Peatland management and restoration alters the exchange of methane with the atmosphere. Management of peat soils typically converts them from methane sources to sinks."

Nitrous oxide emissions from agriculture have almost doubled since the 1960s, the report says. Nitrous oxide is a short-lived but potent greenhouse gas; one tonne of nitrous oxide is equivalent to 265 tonnes of CO2 over a 100-year time period.

Nearly two-thirds of the world's nitrous oxide emissions come from agriculture, the report says. Most of these emissions come from the application of nitrogen fertiliser, the report says. Fertiliser application on crops has increased nine-fold worldwide since 1961, it adds.