It is early spring and I step outside to stand in a patch of warm sunshine. The gusts of wind are cool, however, and the palm trees, with their huge green fronds, appear to dance and sway. Beyond them the sky is cloudless and perfectly blue – there seems to be no end to it. On bright afternoons like this, it is easy to forget just how close the darkness of outer space truly is. Our enormous atmosphere, as any astronaut will attest (and many have), is little more than a thin blue line between our planet and the hostile expanse of outer space. It is all that keeps us alive.
There is an analogy making the rounds, and it’s a fairly accurate one, which proposes this: if you take an apple and imagine for a moment that it represents Earth, then the thickness of that apple’s skin in relation to the rest of the apple is roughly proportional to the thickness of Earth’s atmosphere in relation to the planet itself. Our atmosphere is a lot smaller and far more delicate than it seems. Yet, we are inflicting a staggering amount of damage.
‘We are adding roughly 40 billion metric tons of CO2 pollution to the atmosphere per year,’ said Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Association (NOAA), in an announcement by the organisation in June 2021. ‘That is a mountain of carbon that we dig up out of the Earth, burn, and release into the atmosphere as CO2 – year after year.’ In that same announcement, the NOAA revealed that atmospheric CO2 measured at its Mauna Loa, Hawaii, observatory had just peaked at a record high of 419 parts per million – this was despite an entire year of lockdowns and transport disruptions brought about by the COVID-19 pandemic. We have now reached a point where atmospheric CO2 concentrations haven’t been this high since the Pliocene well over four million years ago.
As these emissions continue to rise, so too will Earth’s temperature. That’s because anthropogenic emissions of greenhouse gases, including CO2 but also nitrous oxide and methane, accumulate in the upper atmosphere where they trap heat radiating off Earth’s surface, preventing it from dissipating into outer space. According to the 2020 Global Climate Report by the NOAA, ‘the global annual temperature has increased at an average rate of 0.08°C (0.14°F) per decade since 1880 and over twice that rate (+0.18°C / +0.32°F) since 1981’. The seven warmest years on NOAA’s records have occurred since 2014. As it stands, the average global temperature is now just over 1°C higher than pre-industrial temperatures. The Intergovernmental Panel on Climate Change currently anticipates that, based on our current trajectory, the global temperature average will likely reach a 1.5°C rise over pre-industrial temperatures by 2052 at the latest, and this may even occur as early as 2030.
Rising temperatures increase the likelihood of severe weather events. As the University of Queensland’s Andrew Borrell explained in 2017 in The Conversation:
As temperatures rise, rainfall patterns change. Increased heat also leads to greater evaporation and surface drying, which further intensifies and prolongs droughts. A warmer atmosphere can also hold more water – about 7 per cent more water vapour for every 1°C increase in temperature. This ultimately results in storms with more intense rainfall.
This is already happening, he goes on to say: ‘A review of rainfall patterns shows changes in the amount of rainfall everywhere.’
Rising temperatures are also contributing to something that could be seen as a global circulatory problem. The heat we get from the sun is terribly uneven on any given day and in any given season, largely due to the planet slowly orbiting the sun while rotating on a tilted axis. Oceanic circulation patterns play an important role in climate regulation by distributing that heat, providing a buffer from extreme weather events. Massive oceanic gyres made up of long-range currents provide this service by acting as large conveyor belts carrying warm water from the tropics to the poles and carrying cool water back again. They are further aided by a support cast of thousands of smaller currents and eddies. When made visible with heat-sensitive imaging, they can be seen to flow and spiral across the oceans in a way that very much resembles the swirling sky in Van Gogh’s Starry Night. They are beautiful and far more fragile than many of us realise. Ocean and air temperatures play important roles in the integrity of these circulation patterns, and even small temperature increases can lead to perturbations and slowdowns that ultimately undermine all that useful energy distribution. The weakening of these currents and gyres – and there is evidence that this is already underway – leaves life in the oceans and on land in an increasingly vulnerable situation.
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All organisms have boundaries within which they can survive, and even narrower limits within which they can thrive. Plants are no exception. High temperatures and periods of excessive heat can affect a plant’s ability to successfully complete its full life cycle, especially the reproduction stage.
Take photosynthesis, the process upon which almost all life on Earth depends in one way or another. Photosynthesis is an exquisitely heat-sensitive physiological process. At its most fundamental level, it comprises a series of chemical reactions involving proteins, cell membranes and the transport of molecules, which only function properly within a moderate temperature range. As temperatures rise, the entire process becomes less efficient. The enzyme Rubisco has often been called the most important enzyme in the world because it carries out the central task of converting CO2 into sugar during photosynthesis. This is high praise indeed, especially considering just how sloppy it is at its job. Even under ideal temperatures, Rubisco will accidentally grab an oxygen molecule (O2) instead of a CO2 molecule around 20 per cent of the time. As temperatures rise and molecules move more quickly, Rubisco makes this mistake more often. Even under ideal conditions Rubisco is hampered by sugar molecules that get stuck its molecular machinery, so it relies on a helper enzyme to clear them away. It turns out that this enzyme, called Rubisco activase, is particularly heat-sensitive and has trouble functioning as temperatures rise. As Rubisco activase shuts down during heat-stress, so too does Rubisco and, by extension, the plant’s ability to photosynthesise.
Individual plants cannot scurry out of the hot sun in search of shade or a cool drink, even if such things were available nearby. Instead, plant populations in warmer climates evolved a variety of ways to mitigate heat stress. Some varied their leaf shape, angle, quantity or colour. Some found ways to produce more cellular structures for respiration or more-efficient mechanisms for water uptake and circulation. Some species became more sensitive to light, enabling them to grow better in shade. Many species used a variety of such approaches in order to keep functioning. Because of this, plants vary somewhat in their upper-temperature limits – but there is always a limit. So, too, with the evolution of drought- and flood-tolerance mechanisms. Plants, by and large, did not evolve to live long term at the extremes. Physics always has the last word.
Rising temperatures and an increasing incidence of extreme weather events present a problem for all plants, whether they are in the largest of forests, the smallest of wild glades, or in a crop field. Many crops are especially vulnerable to climate change. Most varieties of rice, wheat, barley and maize are susceptible to drought stress. There is also ample evidence that even short periods of high temperatures disrupt seed development in cereals, which in turn reduces seed quality and yield. When such seeds are used for planting the next crop, they are less likely to germinate. Even if they do germinate, they do so more slowly than healthy seeds and the resulting seedlings are more likely to fail.
Of course, to predict the impact this will have on the world’s food supply, and to respond appropriately, it’s important to realise that climate change is manifesting in many different ways. I’m told this by Mariana Yazbek, the genebank manager at the International Center for Agricultural Research in the Dry Areas in Lebanon. ‘When we talk about climate change, we’re talking about a series of different variations of the conditions in different parts of the world. So it’s not going to be drier everywhere, it’s not going to be wetter everywhere, it’s not going to be hotter everywhere,’ she tells me. In the Middle East it is indeed getting hotter, with temperatures predicted to rise by as much as 2–4°C, she says. ‘We’re talking about drier seasons and we’re talking about also unpredictable rainy seasons. In this part of the world we have a rainy season that’s three, four, five months [long]. Now it’s becoming completely unpredictable and we’re starting to see that.’ The rainfall is also becoming more intense, causing severe flooding. Such extreme rainfall events bring major cities to a standstill and devastate crops in the region, crops that may be drought-susceptible but aren’t fond of being submerged, either.
Extreme rainfall events are a problem even for crops that usually like a lot of water for growth, says Deepak Ray, a senior scientist at the University of Minnesota’s Institute on the Environment. ‘So rice, we think that it loves water, but if you have too little of it or if you have too much of it, it dies,’ he tells me. Rice plants are very sensitive plants that require water yet have a water tolerance limit, he says. While there are a number of flood-tolerant rice varieties, many still struggle if flooding lingers for days or weeks on end. The problem is that a great deal of the world’s rice is grown in tropical regions, precisely where more intense wet weather is ramping up, from increased rainfall to stronger typhoons.
Plants weakened by environmental stressors also have more trouble defending themselves against pathogens and pests. While some of these threats are themselves struggling with climate change, a number of them are getting a boost, especially in places where the conditions are becoming warmer and wetter.
The impact of climate change on global food security isn’t just a future scenario. It’s already here. According to Ray and his colleagues, it has been happening for a number of years now. In a recent study published in the journal PLOS One, they examined global crop productivity for ten crops that supply most of the world’s calories: wheat, rice, maize, barley, soybeans, sorghum, cassava, sugarcane, rapeseed and oil palm. Specifically, they analysed how yields varied in response to temperature and precipitation. With decades of data gathered from all around the world, they were able to get an idea of what kind of crop yields to expect under different weather conditions. Then they did a counterfactual analysis, in which they predicted what crop yields would look like if average weather patterns seen prior to 1974 had simply continued. They then compared that with actual global crop yields. They found that, while in some parts of the world crop production increased at a slightly better rate than the historical average, in most places it was worse, amounting to a global shortfall of around thirty-five trillion calories compared to what it could have been.
Ray clarifies that crop yields are still rising gradually each year, but ‘it could have been steeper’. He says that, due to climate change, ‘something got shaved off. If you had a historical climate – no change – then we would have received that thirty-five trillion calories.’
Meanwhile, we really need to double global crop production. Michael Purugganan explains that, although global crop yields are currently increasing incrementally by about 1 per cent a year, ‘the problem is to meet our population growth, we have to increase our yield 2 per cent a year, so we have that gap that we are struggling to meet if we’re to feed the world by the end of the century or at least to when the population stabilises’. That puts us in a precarious situation. ‘What people don’t seem to understand’, Purugganan tells me, ‘is that our ability to secure our food supply is an existential issue for humans. We’re one or two really bad harvest years away from a large-scale food security issue.’ By this, he clarifies, he means widespread malnutrition and famine.
We’re now dependent on a very small number of crop species. This isn’t good for global nutrition, and those crops are susceptible to climate change. ‘I think there’s a general understanding that the Green Revolution was a lifesaver,’ says Charlotte Lusty. ‘It underpins civilisation as we know it.’ But it involved a trade-off. ‘Before the dwarf varieties that came out as part of the Green Revolution … there was a lot of diversity out in farmer’s fields, and there were different approaches to agriculture with less fertiliser, pesticides and machinery involved,’ she tells me. It’s becoming increasingly important to breed climate-resistant crops, but Lusty notes that there is not going to be a one-size-fits-all solution. ‘It’s not going to be possible just to develop a climate-resilient variety of wheat,’ she says. Instead, resilience needs to be built into the whole food system. ‘I think there’s a very clear need to ensure that food systems are diversified.’
Breeders are already working on this, says Mariana Yazbek. ‘They are [breeding] varieties that are heat tolerant, that can withstand higher temperatures and still produce, that can withstand lower water availability,’ she says, adding that they are searching for a variety of useful traits. ‘But sometimes, they don’t find that trait [in] the material they’re working with.’ And therein lies the rub. You can’t just pull traits out of thin air – they need to come from somewhere. A plant with the desired trait could have one or more genes that simply aren’t present in the crop variety the breeder wants to improve, or it might have variations in the same genes, or just different combinations of the same genes. It’s like Lego, says Yazbek, explaining that you can build many different things by using the same basic components in different combinations.
Where do you find these traits and the genetic differences that underpin them? ‘Crop wild relatives’ are a good place to look, says Yazbek, precisely because whatever big climatic changes are coming, there are species out there that have been through it before. ‘We have the wild relatives with all the diversity, all the adaptation, all the richness that survived millions of years, and which we know has the potential to save us,’ she tells me. ‘We know they’ve been through a lot, so if it was a few degrees hotter, [a] few degrees cooler, more water, less water, they have been through that!’
But didn’t uniquely acclimated species die out when conditions changed over time? Not all of them, says Yazbek. In the same way that future climate change will manifest differently in different parts of the world, so too did past climate change. The ice age ended but there are still glacial areas. Global warm periods rose and fell over many millennia but we still have tropical forests. On an even smaller scale, there are microclimates that persisted and enabled some plant species to survive. A species that enjoyed widespread territory during a past climate might now be found on the eastern side of a small hill in Lebanon, says Yazbek. ‘They’re very old and they’re there.’
Crop wild relatives are full of useful genes that evolved under environmental pressures over vast periods of time. They have a wealth of genetic diversity that is going to be very useful to plant breeders, so it will be critical to find and preserve as many crop wild relatives as possible. You never know precisely which plant is going to have that winning combination of genetic Lego. But time is of the essence because, while they might have survived this long, many crop wild relatives are now under threat. According to Bioversity International, habitat degradation is straining the natural distribution of these species. As agricultural land and urban centres expand, small hotspots of plant diversity are disappearing. For example, a 2020 study published in the journal PNAS examined 600 crop wild relatives native to the United States. Their findings indicated that 28 per cent of those species may be vulnerable, around 50 per cent may be endangered, and 7.1 per cent are likely to be critically endangered.
Genetic erosion is not just a threat to wild biodiversity. Traditional crop landraces are also threatened. These are the local domesticated crop varieties that were developed by our ancestors over thousands of years, says Yazbek, explaining that ‘they have been passing [these crops] from generation to generation, every farmer adding a little bit, making decisions influenced by the environment’. Like crop wild relatives, landraces are a rich source of biodiversity, too, she says. Heat-tolerant, drought-tolerant, flood-tolerant and pest-tolerant landraces could prove to be immensely valuable in the years ahead. The problem is that many of them have vanished from farmers’ fields since the Green Revolution.
But there is good news – they weren’t lost entirely. The Green Revolution came about because people went out looking for useful plant traits that could help stave off widespread famine, says Lusty. From the 1960s to the 1980s, many botanists and breeders around the world went out into the fields and collected seeds, she tells me. ‘[They] were all kind of following in the footsteps of Vavilov. They all went collecting in areas that have since become much more intensified in agriculture and they brought together quite large collections that were the basis of developing improved varieties.’
Remarkably, these collections still exist, hundreds of thousands of seeds, decades old. Since then, scientists have continued in this vein, searching for and preserving traditional crops, collecting their seeds as well as the seeds of crop wild relatives and many other edible plants that are now at risk of disappearing. And it’s not just crops: there is an entire world of plant biodiversity that needs saving. The seed hunters are out there right now, on a hillside in Lebanon, scaling a cliffside in Hawaii, or trekking along a dry riverbed in Australia’s Top End.
The question, of course, is where does one put millions of years of evolution? That’s a lot of genes to save. The answer is that you do what Nikolai Ivanovich Vavilov did. You build a botanical ark – actually, not just one, but many.