深度解析：氣候變遷、土地劣化影響區域衝突和糧食安全？

What does the report say about desertification?

The IPCC land report defines desertification as "land degradation in arid, semi-arid, and dry sub-humid areas, collectively known as drylands, resulting from many factors, including human activities and climatic variations".

Therefore, despite its name, desertification is not "equated to desert expansion", the report notes, but "represents all forms and levels of land degradation occurring in drylands".

The term "drylands" encompasses parts of the world that are defined as dry sub-humid, semi-arid, arid or hyper-arid – indicated by the yellow, orange and red shading in the IPCC map below.

(For a detailed primer on desertification, Carbon Brief published an explainer earlier this week.)

Drylands are home to approximately 38% of the global population, the report says – around three billion people. The largest number of people living in drylands are found in South Asia, followed by sub-Saharan Africa and Latin America. Overall, "about 90% of the population in drylands live in developing countries", the report notes.

Dryland populations are "highly vulnerable to desertification and climate change", the report notes, because their livelihoods "are predominantly dependent on agriculture; one of the sectors most susceptible to climate change".

The drivers of desertification mirror those of land degradation more widely – covered earlier– however, drylands are particularly vulnerable to land degradation because of scarce and variable rainfall as well as poor soil fertility.

Tracking the global extent and severity of desertification is particularly tricky because there is no single way to define or quantify it. Estimates also tend to "vary greatly due to missing and/or unreliable information", the report says.

There are three main three methods of assessing the extent of desertification – expert judgement, satellite images of vegetation change and biophysical models. Together they "provide a relatively holistic assessment", the report says, "but none on its own captures the whole picture".

Nonetheless, "there is high confidence that the range and intensity of desertification has increased in some dryland areas over the past several decades", the report says. "Warming trends over drylands are twice the global average," it adds, and "some temperate drylands are projected to convert to subtropical drylands as a result of an increased drought frequency causing reduced soil moisture availability in the growing season".

The report also has "high confidence" that "risks from desertification are projected to increase due to climate change". For example, under the shared socioeconomic pathway SSP2 ("Middle of the Road") at 1.5C, 2C and 3C of global warming, the population living in drylands and exposed to risks such as water stress, drought intensity and habitat degradation is projected to reach 951 million, 1.15 billion and 1.29 billion people, respectively.

Global warming "is projected to reduce crop yields across dryland areas, potentially reducing local production of food and feed", the report says. Without concomitant increases in agricultural productivity and reductions in food waste and loss, meeting growing food demand will thus likely require expansion of farmland into ever-more marginal areas.

As a primary driver of soil erosion in drylands is cropland expansion, this risks further contributing to desertification, the report warns. It adds that the combination of agricultural productivity declines, changes in food prices and increases in extreme weather events is likely to exacerbate poverty for some dryland populations.

"There is an increasing concentration of poverty in the dryland areas of sub-Saharan Africa and South Asia," the report notes, "where 41% and 12% of the total populations live in extreme poverty, respectively."

Climate change is also likely to "provide an added incentive to migrate" in some places, the report says. And while outward migration could reduce pressure on the land in the short-term, "this can increase the costs of labour-intensive SLM [sustainable land management] practices due to lower availability of rural agricultural labour and/or higher rural wages".

As with other forms of degradation, desertification can both influence climate change and be influenced by it. The figure below summarises some of these feedbacks, with positive effects shown by red arrows and negative effects by blue.

For example, a decline in vegetation (below centre in the figure) can leave the soil more at risk of erosion, increasing the likelihood of sand and dust storms (top right). The effects of this dust "would tend to decrease precipitation" in the local climate (top, right to left), the report says, thus further reinforcing desertification.

The report also details a series of "hotspots" to "present rich experiences and lessons learnt" on desertification. These case studies cover the five topics of soil erosion, afforestation and reforestation through "green walls", invasive plant species, oases in hyper-arid areas, and integrated watershed management.

One example given is the Great Green Wall of the Sahara and the Sahel Initiative. This project aims to "restore Africa's degraded arid landscapes, reduce the loss of biodiversity and support local communities" through "establishing plantations and neighbouring projects…from Senegal on the Atlantic coast to Eritrea on the Red Sea coast".

However, although the initiative is underway in several countries, "the achievement of the planned targets is questionable and challenging without significant additional funding", the report warns.

What are the wider impacts of climate change-driven land degradation?

Research suggests land degradation is already having an impact on the livelihoods of people around the world, particularly those living in vulnerable and poverty-stricken regions.

However, in chapter four the authors are clear that it is difficult to identify the fingerprints of climate change in these processes:

"Unravelling the impacts of climate-related land degradation on poverty and livelihoods is highly challenging. This complexity is due to the interplay of multiple social, political, cultural, and economic factors, such as markets, technology, inequality, population growth, each of which interact and shape the ways in which social-ecological systems respond."

When examining these issues, the report says it has been difficult to isolate climate as a factor from other drivers of land degradation.

Most studies in this area have examined livelihoods of small-scale farmers, and while there is widespread speculation about "potential links" between poverty and climate change due to changes in soil and rainfall, the evidence is currently lacking, it says.

Furthermore, while one study estimated that more than two-fifths of the world's poor live in degraded areas, the report says there is "low confidence" in such figures and notes a need for more research in this area.

However, the report also states that climate change is "frequently noted as a risk multiplier" for both land degradation and poverty. In addition, it acknowledges that in many of the poorest parts of the world, poverty, land degradation and vulnerability to extreme events linked to climate change all go hand-in-hand.

The SPM notes there are "synergies" between poverty eradication and efforts to tackle climate change, desertification, land degradation and food security.

More contentious is the link between climate-related land degradation and migration and conflict. When people are pushed into poverty or have their livelihoods stripped as a result of changes in their environment, one adaptation option is to move to another area – this can be internally or across borders. In theory, this can then add to existing stresses in the new location. Research indicates, for example, that climate-related disasters can be a contributing factor to violent conflict.

While the authors note there is still a need for more evidence about the links between climate-induced migration and conflict due to land degradation, the SPM states:

"Changes in climate can amplify environmentally induced migration both within countries and across borders, reflecting multiple drivers of mobility and available adaptation measures. Extreme weather and climate or slow-onset events may lead to increased displacement, disrupted food chains, threatened livelihoods, and contribute to exacerbated stresses for conflict."

The main report cites a handful of examples of such processes. For example, in both Guatemala and Kenya, declining soil quality has been widely cited as a driver behind migrants heading farther afield to generate income.

Meanwhile, conflicts in both Rwanda and Sudan have been linked by some scholars to land degradation, whereas others have found climate change is a "weak predictor" for armed conflict. As such, the report finds that the evidence from this area is largely inconclusive:

"Studies addressing possible linkages between climate change – a key driver of land degradation – and the risks of conflict have yielded contradictory results and it remains largely unclear whether land degradation resulting from climate change leads to conflict or cooperation."

How can climate change affect food security?

Another major focus point in the report is the ways in which climate change can affect food.

Climate change has "already affected food security" in many world regions, the SPM says.

Warming is affecting food availability by having a direct impact on crop production, says chapter five of the report:

"Observed climate change is already affecting food security through increasing temperatures, changing precipitation patterns, and greater frequency of some extreme events. Increasing temperatures are affecting agricultural productivity in higher latitudes, raising yields of some crops (maize, cotton, wheat, sugar beets), while yields of others (maize, wheat, barley) are declining in lower-latitude regions."

The current impact of climate change on crop yields varies from region to region, the report notes.

In Asia, some parts, including northern China, have seen rice yield increases as a result of regional warming and other factors, such as agricultural innovation, the report says. Crop yield studies focusing on India, meanwhile, have found that warming cut wheat yields by 5.2% from 1981 to 2009, the report says.

In Africa, yields of staple crops such as maize, wheat, sorghum and fruit, including mangoes, have decreased across the continent "in recent years", the report says. "The Sahel region of Cameroon has experienced an increasing level of malnutrition, partly due to the impact of climate change since harsh climatic conditions leading to extreme drought have a negative influence on agriculture."

(To read more about how climate change impacts food around the world, read Carbon Brief's recent interactive article on how traditional dishes could fare if temperatures rise.)

Climate variability can also have a "profound" impact on food availability through phenomena such as El Niño, notes chapter five.

For example, this "occurred during late 2015 to early 2016 when a strong El Niño contributed to regional shifts in precipitation in the Sahel region," the report says. "Significant drought across Ethiopia resulted in widespread crop failure and more than 10 million people in Ethiopia required food aid."

The impact of climate change on food yields is expected to worsen in coming decades, the report says.

The maps below, taken from page 27 of chapter five, shows median yield changes expected in 2070-99, when compared yields from 1980-2010, under a severe climate change scenario ("RCP8.5"). Changes are shown for maize, wheat, soy and rice. On the chart, red shows yield declines of up to 50% while blue shows increases of up to 50%.

It is worth noting that the projections consider the potential impacts of the CO2 fertilisation effect and nitrogen availability stress (both explained in more detail above).

The map indicates that severe wheat losses could occur in South America and parts of sub-Saharan Africa if future climate change is very high.

As well as affecting yields, climate change could alter food quality, the SPM notes: "Increased atmospheric CO2 levels can lower the nutritional quality of crops." Nutrients that could be affected by rising CO2 include protein and zinc, the report says.

Another way that climate change affects food is through boosting pests and diseases. The report finds "robust evidence that pests and diseases have already responded to climate change". It says:

"There is some evidence that exposure will, on average, increase, although there are a few examples where changing stresses may limit the range of a vector [disease carrier]."

Future climate change could also affect livestock production, the report notes. Desertification could affect rangelands where cattle are reared, the report says, while increases in heatwaves could have a "direct impact on animal morbidity, mortality and distress".

Changes to food availability could in turn have an impact on food access and stability via changes to food prices, the report says.

Cereal prices could increase by 1-29% by 2050 as a result of climate change, the SPM says, "leading to higher food prices and increased risk of food insecurity and hunger".

Higher food prices particularly threaten the world's poor, the report notes:

"Decreased yields can impact nutrient intake of the poor by decreasing supplies of highly nutritious crops and by promoting adaptive behaviours that may substitute crops that are resilient but less nutritious…In the developed world, poverty is more typically associated with calorifically-dense but nutrient-poor diets, obesity, overweight, and other related diseases."

How important are socioeconomic changes for future land degradation?

The report emphasises the role that socioeconomic changes around the world will have in determining the planet's future, noting that projections often show these shifts having a larger impact on land use patterns than climate change itself.

The harm generated by the changing climate will not only result from rising temperatures, but also on how humanity's consumption patterns, land management and populations shift alongside it, the report says. This is outlined concisely in the SPM:

"Pathways with higher demand for food, feed, and water, more resource intensive consumption and production, and more limited technological improvements in agriculture yields result in higher risks from water scarcity in drylands, land degradation, and food insecurity."

The "pathways" it refers to are a standard set of "shared socioeconomic pathways" (SSPs), which are used across the IPCC's reports to give a sense of how changes in global society, demographics and economics will interact with climate change.

While there are five SSPs in total, the land report mainly focuses on the first three, with each one offering significantly different outcomes. A Carbon Brief explainer from last year outlines what each of these scenarios means in more detail.

SSP1 is characterised by low population growth, with just seven billion people on the planet by the end of the century. High incomes, reduced overall inequalities and effective land use regulations are also features of this scenario, as well as less meat consumption and food waste.

Based on a continuation of past trends in consumption and technological progress, SSP2 results in medium population growth, hitting nine billion by 2100.

SSP3 meanwhile sees population nearly double that of SSP1, at 13 billion in 2100. This pathway is characterised by low incomes, resource-intensive consumption patterns, barriers to trade and a slow rate of technological change.

Which of these socioeconomic pathways is followed – and the changes in population, consumption patterns and other factors that result – has a large impact on the way land will be used in future, the report says.

The figure below shows how much more (positive numbers) or less (negative numbers) of the Earth's surface would be devoted to forest (blue line and shaded area), bioenergy crops (purple), "natural land" (red), cropland (yellow) or pasture (green) through the 21st century, compared with the levels in 2010.

From left to right, each chart shows how land use will change in scenarios limiting warming to 1.5C by the end of the century. The left-most chart shows this for SSP1, with SSP2 in the centre and SSP5 on the right (this is a resource-intensive, fossil-fuel dependent pathway).

These changing priorities for land use under each different socioeconomic pathway are expected, in turn, to influence how strongly rising temperatures increase risks, the report says.

These influences are illustrated in the figure below, which shows how two very different pathways – SSP1 and SSP3 – will affect climate-related risk.

For example, SSP1 carries only a "moderate risk" of desertification (leftmost column in the chart below) even if temperatures rise as high as 3C. (This is the increase the world is currently on track for given nations' existing climate commitments.)

In contrast, under the SSP3 scenario, the risk of desertification becomes "high" at between 1.2C and 1.5C of warming, and approaches "very high" below 3C (second from left). There is a similar trend for other risks relating to land degradation, such as fires and coastal flooding.

The report notes that the greatest number of relevant SSP studies to date consider how food security will be affected as temperatures rise.

Changes in crop yields and accompanying threats to the food supply have already been attributed to climate change, but again these risks are exacerbated by certain socioeconomic scenarios, it says.

One study estimated that SSP1 would see 100 million people at risk of climate-related hunger beyond 2.5C, while SSP3 would leave over 800 million people vulnerable by 3C.

The authors conclude that addressing cycles of poverty, land degradation and emissions "in a holistic manner" will be necessary to produce sustainable development of the kind exemplified by SSP1 and to avoid the worst outcomes.

For more information, Table 6.2 in chapter six of the report outlines the key challenges faced under different SSPs – including SSP4 and SSP5 – for outcomes including climate change adaptation and mitigation, land degradation and food insecurity.