Global heating now......and then
How does the current chapter of planetary heating, driven by human activities, compare to natural hothouse conditions in Earth history?
One reason why it is taking so long to get people to grasp the scale of the threat posed by contemporary global heating is that the numbers involved seem so insignificant. Many still find it hard to believe that a temperature rise as small as 1.5°C (compared to pre-industrial times)is sufficient to bring about all-pervasive, dangerous, climate breakdown. After all, such a small temperature change would make little difference to the feel of a warm summer day or chill winter night.
The important distinction is that global heating is driving a permanent, all-year round, increase in temperature, which has far more impact on the climate and ecosystems than a transitory hike. Furthermore, the fact that the temperature rise figures quoted are averaged across the globe hides some critical detail.
In particular, heating is not evenly distributed across the planet, so that higher latitudes are heating far more rapidly than the tropics. This has enormous ramifications for the impact of heating as this is where most of the world's ice and frozen ground resides. The consequences for our world and our society of a few degrees temperature rise at the poles, therefore, are hugely greater than for the same rise in the tropics. While meltwater from the collapsing ice sheets drives up sea level at an ever-accelerating rate, the thawing of permafrost allows the methane trapped beneath to vent into the atmosphere. A particularly potent greenhouse gas, methane helps push up temperatures even further in an example of positive feedback.
The speed with which our planet is warming also differs across the land and oceans, with the latter lagging behind significantly. This is because the water that covers more than two thirds of our world is really a colossal heat sink, capable of soaking up a vast quantity of heat that is only slowly reflected in a rise in sea surface temperature. This is enormously fortunate for us, given that the oceans have soaked up more than ninety percent of the heat arising from our greenhouse gas emissions.
Although heating up at a slower rate than the land masses, oceans are getting progressively hotter, so much so that temperature records are being broken every year, 2020 seeing the highest temperature ever recorded. In the past year alone, the oceans have absorbed heat equivalent – it has been calculated - to 630 billion common-or-garden hairdryers running full blast, 24/7, for the same period. A mind-boggling statistic.
Ominously, since 1986, the oceans have been heating at a rate eight times faster than during the preceding quarter of a century. This is important, because the oceans constitute a key part of the global weather machine. Hotter oceans can modify weather patterns that drive drought and floods, influence major climate signals such as El Niño, which have far-reaching impacts on the weather, and increase the occurrence and strength of storm activity, which feeds off ocean heat. It is no surprise that 2020 saw record numbers of major tropical storms rampaging across the Atlantic Basin.
So much for the pattern of global heating arising from human activities, but what about the degree of heating our world has so far experienced and can expect to see in the course of this century? The global average temperature of our world is now (January 2021) more than 1.2°C hotter than it was at the end of the 19th century. Within decades, it is likely that the 1.5°C dangerous climate change guardrail will be breached, while a recent study suggests that, whatever we do in terms of slashing emissions, we are already committed to a global average temperature rise in excess of 2°C, with all that this entails for society and increasingly stressed ecological systems. In the absence of drastic emissions cuts, the temperature of our world – by the century's end - may be a catastrophic 4.5°C hotter (compared to pre-industrial times)and sea level more than 2m higher. There have undoubtedly been other heat spikes during Planet Earth's 4.6 billion-year history, driven by natural variations rather than human action, so how does this current one compare?
A study published recently provides evidence that our planet is now hotter than at any time during the last 12,000 years, and perhaps since the last interglacial period (known as the Eemian) around 125,000 years ago. Then, our world was 1 – 2°C warmer than it is today – in other words the same temperature it is highly likely be by mid-century or even earlier. The climate was hot enough so that Hippos roamed the Thames Valley and sea level, driven by partial melting of the polar ice sheets, was at least 6m higher than it is today. The Eemian climate, however, was not driven by higher carbon dioxide levels, so is not an exact analogue of our current situation.
If we go back further, however, to a time between 11 and 17 million years ago known as the Miocene Climate Optimum, carbon dioxide levels were comparable to what they are now, probably even somewhat lower. Conditions then, therefore, provide a more valid comparison to the situation today. The fact that global temperatures were 2 – 4°C up on pre-industrial values, and sea levels a staggering 20m higher sheds light on the seriousness of our current predicament.
Without doubt, however, it is the rate at which greenhouse gases are accumulating in the atmosphere that really rings the alarm bells. In fact, greenhouse gas levels have not climbed this rapidly since dinosaurs roamed the planet. Fifty-six million years ago, during a time known as the Palaeocene-Eocene Thermal Maximum (or PETM), natural processes resulted in a carbon outburst that pushed global temperatures rapidly upwards by around 5°C, leading to marine extinctions and major changes to environments and ecosystems. Making the most of the accelerated warmth, crocodiles thrived north of the Arctic Circle, while palm trees adorned parts of Siberia.
The scary thing is, carbon - due to human activities – is being released into the atmosphere at more than 10 times the rate of natural emissions during the PETM. This puts us well and truly in uncharted territory, and marks out our time as quite possibly unique. This is a sobering thought indeed. We can build models that seek to shed light on the world of the late 21st century and beyond, and we can look back at past climates for clues as to where our world is heading. When all is said and done, however, we need to recognise that the current situation is close to unprecedented in our planet's unimaginably long existence. As such, in relation to forecasting what sort of world our children and grandchildren will likely inhabit, all bets are off.
Bill's new novel, SKYSEED – an eco-thriller about climate engineering gone wrong – is published by The Book Guild.
Thank you Bill - an astonishing piece, laden with fact and clarity. Can you pop a couple of copies of it in the post, first class, to Washington DC and No: 10, please!