The climate book, p.9
The Climate Book, page 9
The warming of the planet has also altered the speed at which storm systems move across the Earth (translation speed). For the parts of the oceans where we have data, translation speeds have slowed down. And slower-moving storms mean that more rain can be dumped at any single location. Thus, taking together everything that we know from physics, statistics and observations, the storms we see today are more damaging than they would have been without climate change.
Pinpointing the role of climate change on individual extreme weather events is an incredibly valuable source of information for decision-makers as they try to rebuild after disasters and plan for the impacts of tomorrow’s extreme weather. Unfortunately, access to this information is not equal for all. In many cases – for instance, Cyclone Idai, which devastated Mozambique in 2019, or Cyclone Amphan, which hit Bangladesh and India in 2020 – the models are inadequate or inaccessible to scientists from the Global South. Our knowledge of how weather is changing and what our societies are most vulnerable to is dominated by the research and experiences of the Global North. Given the increasing speed with which our climate is warming, these inequalities need to be addressed. Whether a storm turns into a catastrophe ultimately depends on who and what is in harm’s way; and, even if most changes in the climate system are linear, the impacts and damages are absolutely not. Small changes in the climate can have catastrophic consequences. /
Human-made climate change is catastrophic even in a single storm.
A large rainstorm crosses the Irrawaddy Delta in Myanmar in May 2008, four weeks after the river was struck by a storm surge from Cyclone Nargis which killed over 100,000 people.
2.8
The snowball has been set in motion
Greta Thunberg
Maybe it is the name that is the problem. Climate change. It doesn’t sound that bad. The word ‘change’ resonates quite pleasantly in our restless world. No matter how fortunate we are, there is always room for the appealing possibility of improvement. Then there is the ‘climate’ part. Again, it does not sound so bad. If you live in many of the high-emitting nations of the Global North, the idea of a ‘changing climate’ could well be interpreted as the very opposite of scary and dangerous. A changing world. A warming planet. What’s not to like?
Perhaps that is partly why so many people still think of climate change as a slow, linear and even rather harmless process. But the climate is not just changing. It is destabilizing. It is breaking down. The delicately balanced natural patterns and cycles that are a vital part of the systems that sustain life on Earth are being disrupted and the consequences could be catastrophic. Because there are negative tipping points, points of no return. And we do not know exactly when we might cross them. What we do know, however, is that they are getting awfully close, even the really big ones. Transformation often starts slowly, but then it begins to accelerate.
Stefan Rahmstorf writes that ‘We have enough ice on Earth to raise sea levels by 65 metres – about the height of a twenty-story building – and, at the end of the last ice age, sea levels rose by 120 metres as a result of about 5°C of warming.’ Taken together, these figures give us a perspective on the powers we are dealing with. Sea-level rise will not remain a question of milli-, centi- or decimetres for very long. Even if the change takes time, we must realize that this is not something we can adapt to.
The Greenland Ice Sheet is melting, as are the ‘doomsday glaciers’ of West Antarctica. Recent reports have stated that the tipping points for these two events have already been passed. Other reports say they are imminent. That means we might already have inflicted so much built-in warming that the melting process can no longer be stopped, or that we are very close to that point. Either way, we must do everything in our power to stop the process because, once that invisible line has been crossed, there might be no going back. We can slow it down, but once the snowball has been set in motion it will just keep going.
Billions of people all over the world are dependent on the cryosphere, relying on glaciers for drinking water and irrigation. And these are melting too, rapidly. Here we have already passed a number of irreversible tipping points which will bring enormous challenges in the decades to come. The Himalayan glaciers, also known as the Third Pole, are particularly crucial, as 2 billion people across Asia rely on them for their water supply. These glaciers are currently melting at an exceptional rate; one landmark study, requested by the eight nations spanning the area and carried out by 200 scientists, finds that even if we limit warming to 1.5°C, one third of the ice mass will be lost.
Not only are we losing this vital resource, we are doing so at a pace which in itself is a problem – because the faster melting speed is making us used to unnaturally high levels of water flow. When all that water starts to run out, we will be in even more trouble. Our infrastructure and societies were built for the Holocene, which is now becoming a geological epoch of the past. The world we used to safely inhabit no longer exists. /
Sea-level rise will not remain a question of milli-, centi- or decimetres for very long. Even if the change takes time, we must realize that this is not something we can adapt to.
2.9
Droughts and Floods
Kate Marvel
The Earth does not, in general, make its own water. It doesn’t have to. Plenty arrived from space at the planet’s formation and, essentially, the same amount has remained ever since. Billions of years from now, when the sun burns through its own store of fuel and dies, the Earth’s moisture will disappear into space, ready to water the surface of some distant planet.
What this means is that the water we drink is the same stuff that quenched the dinosaurs’ thirst and nourished the first stirrings of life on the young world. It morphs from ice to liquid to vapour and back again, rises from humid forests and sinks into cold ocean abysses, moves from the tropics to the poles and back. Sometimes, when the planet wobbles a little in its orbit, some of the water finds itself locked in glacial ice for an aeon or two. When the ice age ends, it escapes in a fresh torrent that pours into a growing ocean. On shorter timescales – afternoons, months, human lifetimes – it cycles from the ocean or land to the sky and back, not created, not destroyed, always changing.
Shape-shifting is tiring work. It takes energy to turn liquid into vapour, which is why on hot days your body makes you wet and clammy. Evaporation wicks energy away from the surface and up into the sky. Condensation warms the upper atmosphere, which in turn spits heat outwards to cold space. Water in vapour form is invisible, but the sky is visibly painted with white and grey clouds, collections of tiny liquid drops and ice crystals. The Earth sweats in the heat. The cold upper atmosphere wraps itself in a blanket of cloud. Everything is in balance, until everything is disturbed.
As the temperature increases, the world sweats more. The air demands water from the surface, which yields up its moisture to the thirsty sky. The oceans can easily handle the increased demand. But on land, the water is stored in soil like a sponge. Even in years with average rainfall, the greedy air can suck the lifeblood from the surface, leaving it arid and dead. The North American south-west is experiencing the worst megadrought on record, with more drying to come. Southern Europe, the Levant and south-western Australia are drying, too, as expected when temperatures rise. Drought is the consequence of a planet desperate to cool itself off.
In the process of evaporation, liquid morphs into vapour: colourless, odourless, but far from weightless. There are 10 million billion kilograms of water vapour in the atmosphere, pushing up and down and to the side, exerting pressure everywhere. Eventually the pressure becomes unbearable, and some of that vapour escapes the sky, condensing back into liquid. The threshold where this happens increases rapidly with temperature: hot air can hold more water vapour. There is a bank of water in the sky, receiving credits of vapour, spending debits of rain and saving a little in reserve. As the temperature increases, the reserves pile up. There is more moisture in a warming sky, a 7 per cent increase for every degree Celsius of warming. On a hotter planet, when it rains, it pours. A warmer world will suffer from drought but, by the cruel logic of the water cycle, it will flood too.
Our worsening droughts and our catastrophic floods are characteristic fingerprints of human interference, a record of our post-industrial existence etched in the water flows of the planet. Attribution science has now advanced so much that we can quantify the human contribution to individual downpours and droughts. But our fingerprints are visible on much larger scales than that, etched on sky, sea and land. Satellite observations show long-term shifts in rainfall patterns that are corroborated by the ocean. The waters of the Southern Ocean and the north Atlantic have freshened as the rainfall in those regions has increased, while the Mediterranean and subtropical seas have become saltier under drier skies. On land, old trees give long context to our current moment. Their inner rings tell a story of wet and dry years past, of changing moisture in the soil that feeds them.
Together, the tree rings of the world form a pattern, a record of moistening and drying that stretches back for centuries. These changes are natural. But now, something unnatural is beginning to emerge. As we look at the rings of the last century, we see drying soils in the thin rings of thirsty trees. It’s not unusual for the American south-west to be dry, or the Mediterranean, or Australia. Droughts would happen even in a world without us. But it is unusual for all of these places to be dry at once. Nature can’t do that. But we can.
We now live in a world largely of our own creation. What will we do with it? We will not sit back to wait patiently for calamity. We will rethink the world we’ve made. We will draw our energy from the sun and the wind that drive the dance of water from surface to atmosphere and back. We will endure and change, like the water on which we depend. We must. /
2.10
Ice Sheets, Shelves and Glaciers
Ricarda Winkelmann
December, 2010: minus 32°C. Our research vessel has reached 71°07 S, 11°40 W – Antarctica. It’s 4 a.m. and as bright as day. I look out at the ice shelf in front of us, which is protruding roughly 30 metres from the ocean water. I am stunned by its beauty, by the complex structures within the ice, and I can barely wrap my mind around its vastness: covering almost 14 million square kilometres, it is, in many places, more than 4,000 metres thick. If all this ice were to melt, sea levels would rise by almost 60 metres worldwide. Looking up, I think to myself: much of this ice was formed hundreds of thousands of years ago. Humans, on the other hand, only set foot on Antarctic ice in the early twentieth century. How is it possible that, in this short time, we have become the dominant force determining the future evolution of this majestic giant?
I will never forget this moment from my first scientific expedition to Antarctica. It was then that I sensed what it truly means to have entered the Anthropocene – that humans have become a geological force.
Increasingly, our actions are affecting all parts of the Earth system, including the planet’s two ice sheets, in Greenland and Antarctica. Over the past few decades, both ice sheets and their surrounding ice shelves – which are like floating tongues of ice jutting into the ocean – have been losing mass at a rapidly accelerating rate. In total, 12.8 trillion tonnes of ice were lost between 1994 and 2017. To put this into perspective, 1 trillion tonnes of ice can be envisaged as an ice cube measuring 10 cubic kilometres, taller than Mount Everest.
In the future, the ice sheets are expected to become the largest source of sea-level rise. Because of their massive size, even modest losses from them can significantly increase the risk of flooding in coastal communities, with severe consequences for society, the economy and the environment.
Drastic changes in the polar regions are already beginning to unfold. In 2020, temperatures hit a record high in both polar regions, reaching +18.3°C on the Antarctic Peninsula and +38°C in the Arctic. In 2021, two near-record melt events occurred on the Greenland Ice Sheet – following a series of extreme melt events in the years 2010, 2015 and 2019. On the other side of the planet, the largest iceberg in the world calved from the western side of the Ronne Ice Shelf in the Weddell Sea. Analysis of satellite images revealed additional large icebergs breaking off at the edge of the ice shelf next to Pine Island Glacier, leading to further acceleration of what is already one of the fastest-moving glaciers in Antarctica.
While these are only snapshots in time, they reveal the radical and impactful shifts currently occurring in and around the ice sheets. The polar regions are our planet’s most efficient early-warning systems for progressing climate change – and these early-warning systems are now raising the alarm.
And we’d better listen to this alarm: with unmitigated climate change, we will push the ice sheets further and further out of balance, potentially unleashing self-perpetuating processes that cannot effectively be halted.
One of these self-perpetuating processes, or positive feedbacks, is linked to melting at the surface of the Greenland Ice Sheet: with increasing melt, its surface is slowly sinking to lower heights. At lower elevations, the air is generally warmer, which can in turn lead to more melt, making the surface descend into even warmer air masses, causing even more melt, and so on. Once a critical temperature is exceeded, this melt-elevation feedback could lead to sustained ice loss until Greenland would eventually be almost ice-free.
Because it is colder in Antarctica than in Greenland, it is not so much the surface melt that threatens the stability of the Antarctic Ice Sheet but rather what is going on underneath. Much of the observed ice loss in Antarctica happens as a result of the melting of the floating ice shelves that surround the continent. These get thinner when they come into contact with warmer ocean waters, causing the continental ice further inland to accelerate towards the ocean, and potentially leading to self-perpetuating ice loss.
These positive feedbacks are why both ice sheets are considered tipping elements in the Earth system. Once they are close to a critical warming threshold, or tipping point, a tiny perturbation can suffice to trigger abrupt, widespread, unstoppable ice loss.
The risk of crossing such a tipping point increases starkly when exceeding global warming of 1.5–2°C. Above these temperature levels, large parts of the Greenland and Antarctic ice sheets will be lost, and long-term persistent sea-level rise of several metres will be unavoidable. Even if temperatures eventually were to sink again, cooling well below today’s temperature would be required to regrow the ice sheets to their present-day size. In other words, parts of the ice sheets, once lost, might be lost forever. /
2.11
Warming Oceans and Rising Seas
Stefan Rahmstorf
In 1987 one of the great pioneers of oceanography sounded a warning in the top scientific journal Nature:
The inhabitants of planet Earth are quietly conducting a gigantic environmental experiment. So vast and so sweeping will be the consequences that, were it brought before any responsible council for approval, it would be firmly rejected. Yet it goes on with little interference from any jurisdiction or nation. The experiment in question is the release of CO2 and other so-called greenhouse gases to the atmosphere.
Wallace (Wally) Broecker wrote these words, and I was lucky enough to work with him for years on the Panel on Abrupt Climate Change before he sadly passed away in 2019. Here, I’ll take a look at the consequences of this ‘gigantic environmental experiment’ for the physical aspects of the ocean – ‘physical’ here meaning physics rather than marine biology or chemistry, which are covered elsewhere in this book.
The warming ocean
The oceans have absorbed over 90 per cent of the extra heat on our planet that has been trapped by increasing levels of greenhouse gases. That’s not because the oceans heat up more than the air but because more energy is required to heat water than to heat air (in other words, water has a much larger heat capacity). The oceans absorb this heat at their surface, which sees the highest increase in temperature; it penetrates more slowly into the ocean’s depths. Ocean heat content is increasing at a rate of 11 zettajoules per year, twenty times the amount of energy used by humans.
Despite the oceans absorbing 90 per cent of the additional heat, sea surface temperatures have risen only about half as much as air temperatures over land: by 0.9°C since the late nineteenth century, compared to 1.9°C for temperatures over land (Fig. 1). Given that 71 per cent of the Earth is covered by ocean, that makes for a global average warming of 1.2°C.
Changes in global sea surface temperature and air surface temperature over land
Figure 1: Temperature anomalies for sea ice regions are calculated separately and not shown.
By the time global warming has reached 1.5°C, temperatures over land will have warmed by approximately 2.4°C. So when we talk about the ‘global average temperature’ we make the impact of warming on us land-dwellers appear much smaller than it actually is. However, the relatively large heat capacity of the ocean does mean that our planet takes time to warm up and is lagging behind the equilibrium warming that will eventually be reached.
