Throughout my career a couple of the most regularly asked questions I’ve had from friends and family are:
Is climate change real?
And it often goes with a second question
How can we tell humans are causing it?
Over the last few years, the first question has been asked less, and the second one more. And recently there’s been a switch from a lot of people to the more important question: What can we do to stop climate change?
The answer to all three, however starts with the fact we have to understand what can cause climate change, both naturally and through human activity.
So in short climate change is real. And yes, humans are causing it, and here is an introduction to how we know this with such certainty. We hope this section will be useful and will also inform other posts about what we can do as horse owners.
To understand how we are causing a climate crisis, we first have to understand natural climate change. It is true, that climate has varied substantially in the past, with the most recent period in Earth’s history, the Quaternary, which covers the last 2.6 Million Years, having cycles of cold Ice Ages called ‘Glacials’ interrupted by warm ‘Interglacials’. We are currently in an Interglacial so climate should be relatively warm, but also relatively stable, and as we explain later the current warming can’t be explained naturally.
Large scale cycles of glacial and interglacial conditions are driven by changes in the way the Earth orbits around the sun, which influence climate on long timescales. There are different cycles caused by changes in the shape of the orbit and the wobble of the Earth around its axis and they cause 22,00, 41,000 and 100,000 year cycles. These orbit subtly change the distribution of the Sun’s energy around the globe, it is called insolation, and are the first part of what drives long term climate change. In fact we can detect these signals over millions of years in long marine sediment records.
There is an important problem with this explanation alone and that is simply the changes in amount and distribution of the energy from the Sun can’t explain the scale of climate change we see, and that needs what we call feedback mechanisms. These are aspects of the Earth’s climate and environment that when triggered in a particular way can influence cooling or warming, across particular region or the whole planet.
So let’s start with the big stuff, the movement of the tectonic plates over the surface of the Earth. This process has been going on for billions of years, re-configuring the position of oceans and continents, but a world that looks broadly like ours now took until around 10-15 million years ago. The collision of continental plates including the Eurasian, Indian and African plates leads to the building of mountains. Rocks in these elevated mountains weather and erode and during this process CO2 is drawn out of the atmosphere. This process of weathering has been gradually cooling the global climate for around 15 million years1, but about 2.5 million years ago cooling temperatures reached a point where ice could form in Greenland and the cycles of ice ages and interglacials were able to begin. For the last million years we have been experiencing interglacials around every 100,000 years.
Once were in an ice age world, more feedback mechanisms become important in the rapid switches between warm and cold. For example, CO2 and Methane being locked up or released from ice or permafrost. Our current interglacial started abruptly 11,600 years ago as several feedback mechanisms tipped us out of the last ice age.
Overlain over the long term climate changes there are also forcing factors that occur on much shorter timescales. These include sea ice extent, freshwater release from melting ice, and they can lead to variations in ocean circulation, on abrupt timescales (few hundred to a few thousand years). A nice example is that even in our current stable interglacial, meltwater pulses from receding ice sheets have caused cooling of a few hundred years. In fact, there’s quite a bit of interest in how melting ice sheets may destabilise ocean circulation in the future, causing cooling in Europe but even more intense warming in the Southern Hemisphere and devastating changes in weather patterns. Additionally, on similar timescales, solar activity can vary due to changes in the suns output – we see these as changes in sunspot activity. This generally follows an 11-year cycle of higher and lower sunspot levels, but on longer terms, we see periods of around 100 years when solar activity is low and this occurs when sunspot numbers are low. You may have heard of a period called the Little Ice Age (approx. 1500-1800AD), when people were ice skating on the River Thames and this period included three intervals of low sunspot and solar activity. Interestingly the rate of warming in the transition from the last Ice Age to the current interglacial was similar to what we are now doing to the planet, except this time it shouldn’t be happening, we should be in a relatively warm, stable interglacial.
In our current interglacial, climate is also influenced by volcanic activity – why you ask? Because as a large (usually tropical) eruption occurs, sulphate particulates are released into the Earth’s stratosphere. These stick around for 2-3 years and can result in short term global cooling and also changes in regional weather patterns. Large enough eruptions don’t occur that often. The last big one was the Pinatubo eruption in 1991, which cooled the earth for a couple of years and led to, amongst other things, increased arctic sea ice extend. In fact, polar bears from those years where known as the Pinatubo bears, as they had such a good start in life!
What’s really interesting, is that because we have good measurements of these changes from past records e.g. from deep ice cores, marine sediments, tree rings and long lake records, we can understand this signal and decouple it from human influences. If you want to know more, some good starting points are available on Lorraine Lisiecki’s page for links to long term changes and lots of great details are also available from Antarctic Glaciers.
Why does all this mater? If we want to work out how much humans are influencing the Earth’s climate, and what might happen if we don’t stop, we need a good understanding of natural climate mechanisms. For example, there has previously been debate on how much natural changes in solar activity were influencing the warming we have seen during the industrial period. With some arguing that global warming could be explained by natural phenomena, however, the last few solar cycles have been weaker, while temperatures have continued to rise. Most of the hottest years have been recorded in the last decade, with 2015, 2016, 2019 and 2020 top2.
So how are humans driving climate change?
You’ve probably heard we are driving climate change through greenhouse gas emissions (mainly carbon dioxide and methane), which retain heat in the atmosphere.
CO2 are over 400 ppm, and are nearly twice as high as every interglacial we can measure from Ice cores3 meaning we are completely out of the natural range for the last 800,000 years? What this is driving as the IPCC lays out4 clearly is widespread, rapid and intensifying climate change.
But what about all that natural climate variability, well here is the really worrying bit. We know the Earth’s climate system is sensitive to minor changes in forcing and feedback mechanisms can lead to tipping points, where we move into a new state. The danger is that if we move beyond around 1.5°C of warming we start to get to the point where we get changes that are hard to reverse. The sobering thought is that we are not talking about 1.5°C from now, but above levels from before 1900AD and we have already warmed around 1.09°C of our allowance and there is very little time left.
1. Rugenstein et al. 2019, Nature volume 571, pages 99–102.
2. https://public.wmo.int/en/media/press-release/2020-was-one-of-three-warmest-years-record