Current greenhouse gas concentrations could warm the world 3-7℃ (and on average 5℃) over coming millennia. That’s the finding of a paper published in Nature today.
The research, by Carolyn Snyder, reconstructed temperatures over the past 2 million years. By investigating the link between carbon dioxide and temperature in the past, Snyder made new projections for the future.
The Paris climate agreement seeks to limit warming to a “safe” level of well below 2℃ and aim for 1.5℃ by 2100. The new research shows that even if we stop emissions now, we’ll likely surpass this threshold in the long term, with major consequences for the planet.
What is climate sensitivity?
How much the planet will warm depends on how temperature responds to greenhouse gas concentrations. This is known as “climate sensitivity”, which is defined as the warming that would eventually result (over centuries to thousands of years) from a doubling of CO₂ concentrations in the atmosphere.
The measure of climate sensitivity used by the Intergovernmental Panel on Climate Change (IPCC) estimates that a doubling of CO₂ will lead to 1.5-4.5℃ warming. A doubling of CO₂ levels from before the Industrial Revolution (280 parts per million) to 560ppm would likely surpass the stability threshold for the Antarctic ice sheet.
As the world warms, it triggers changes in other systems, which in turn cause the world to warm further. These are known as “amplifying feedbacks”. Some are fast, such as changes in water vapour, clouds, aerosols and sea ice.
Others are slower. Melting of the large ice sheets, changes in the distribution of forests, plants and ecosystems, and methane release from soils, tundra or ocean sediments may begin to come into play on time scales of centuries or less.
Other research has shown that during the mid-Pliocene epoch (about 4.5 million years ago) atmospheric CO₂ levels of about 365-415ppm were associated with temperatures about 3–4 °C warmer than before the Industrial Revolution. This suggests that the climate is more sensitive than we thought.
This is concerning because since the 18th century CO₂ levels have risen from around 280ppm to 402ppm in April this year. The levels are currently rising at around 3ppm each year, a rate unprecedented in 55 million years. This could lead to extreme warming over the coming millennia.
More sensitive than we thought
The new paper recalculates this sensitivity again – and unfortunately the results aren’t in our favour. The study suggests that stabilisation of today’s CO₂ levels would still result in 3-7℃ warming, whereas doubling of CO₂ will lead to 7-13℃ warming over millennia.
The research uses proxy measurements for temperature (such as oxygen isotopes and magnesium-calcium ratios from plankton) and for CO₂ levels, calculated for every 1,000 years back to 2 million years ago.
Some other major findings include:
The Earth cooled gradually to about 1.2 million years ago, followed by an increase in the size of ice sheets around 0.9 million years ago, and then followed by around 100,000-year-long glacial cycles.
Over the last 800,000 years, and particularly during glacial cycles, atmospheric greenhouse gas concentrations and global temperature were closely linked.
The study shows that for every 1℃ of global average warming, Antarctica warms by 1.6℃.
So what does all this mean for the future?
Global warming past and future, triggered initially by either changes in solar radiation or by greenhouse gas emissions, is driven mainly by amplifying feedbacks such as warming oceans, melting ice, drying vegetation in parts of the continents, fires and methane release.
According to the new paper, such greenhouse gas levels are committing the Earth to extreme rises of temperature over thousands of years, with consequences consistent with the large mass extinctions.
The IPCC suggests warming will increase steadily as greenhouse gases increase. But the past shows there will likely be abrupt shifts, local reversals and tipping points.
Abrupt freezing events, known as “stadials”, follow peak temperatures in the historical record. These are thought to be related to the Mid-Atlantic Ocean Current. We’re already seeing marked cooling of ocean regions south of Greenland, which may herald collapse of the North Atlantic Current.
As yet we don’t know the details of how different parts of the Earth will respond to increasing greenhouse gases through both long-term warming and short-term regional or local reversals (stadials).
Unless humanity develops methods for drawing down atmospheric CO₂ on a scale required to cool the Earth to below 1.5°C above pre-industrial temperature, at the current rate of CO₂ increase of 3ppm per year we are entering dangerous uncharted climate territory.
Tomorrow, delegates from more than 190 nations will begin an 11-day meeting in Montreal to determine the final form of a scheme to reduce greenhouse emissions from the aviation industry.
The meeting – the latest in a series of three-yearly summits held by the International Civil Aviation Organisation (ICAO), the United Nations agency tasked with reducing aviation emissions – is poised to decide on a scheme that would ultimately make it mandatory for most airlines from member countries to buy carbon offsets for their flights.
The resolution would fill a key gap in global climate policy. The Paris climate agreement, brokered last December, makes no mention of aviation emissions, despite having featured these in earlier drafts.
Earlier this month, the ICAO Council issued the final draft of a resolution text to be considered – and, presumably, after some debate, approved – at the Montreal meeting.
In its current form, questions will be raised over the scheme’s effectiveness, not least because it won’t become mandatory until 2027 – and even then not for all carriers. But these loopholes make it more likely that the plan will be adopted.
Mandatory offsetting (in the future)
The planned carbon offsetting scheme set out in the draft resolution would begin with a pilot phase running from 2021 to 2023, involving states that have volunteered to participate. These states will have some flexibility in determining the basis of their aircraft operators’ offsets.
The purpose of this pilot phase is not really clear, and some aviation industry organisations, such as the Air Transport Action Group, regard it as unnecessary.
A first “formal” phase from 2024 to 2026 would apply to states that voluntarily participate in the pilot phase, and again would offset with reference to options in the resolution text. The main difference between the pilot and first phases is that, for the pilot phase, states can determine the applicable baseline emissions year.
A second, mandatory phase would only operate from 2027 to 2035 and would exempt the least developed countries and those with the smallest proportion of international air travel.
There are also exemptions based on the routes themselves. While the rules would apply to all flights between countries covered by the offsetting requirements, they will not apply to flights that take off or land in a non-member state.
Offsetting the issue
Then there are the well-publicised problems with the whole concept of carbon offsetting. Most countries and groups of countries (and ICAO is a group of countries) have ignored offsets in favour of mechanisms such as emissions trading schemes or carbon taxation – and with good reason.
Offsets, which by definition simply move emissions from one source to another, have little net effect on emissions. As such, offsets could be viewed as a diversion from regulations that genuinely encourage emissions reduction, such as carbon pricing. The Paris Agreement does not directly rely on offsets because all governments recognise that it’s collective, substantive action that counts.
What is really needed is a policy that motivates major industrial sectors – aviation included – to cut emissions and use resources more efficiently. Market-based mechanisms offer the best way to apply the price pressure needed to drive such a change.
The question in designing any market-based mechanism is whether to base it on quantity or price. A quantity-based instrument is an ETS, the most common example of which is a cap-and-trade system; a price-based instrument is a carbon tax.
ICAO has chosen neither of these options. Instead, it has chosen a system of voluntary and then mandatory carbon offsets, with all their attendant problems.
An analysis by Carbon Brief has found that even if the aviation industry meets all of its emissions targets, by 2050 it will still have consumed 12% of the global carbon budget for keeping warming to 1.5℃. This could increase to as much as 27% if the industry misses its targets.
Meanwhile, airlines estimate that air travel will grow by an average of almost 5% each year until 2034, in an industry where low-carbon alternatives are difficult to find.
It is perhaps good news, then, that three weeks ago 49 states indicated they were willing to opt into the ICAO’s offsetting scheme in its earliest phase. The following week, in a joint statement, the European Union, Mexico and the Marshall Islands said they would join the scheme. And at G20 talks earlier this month, China and the US offered support.
Brazil, one of the fastest-growing aviation markets, said, however, that it will not join until the mandatory scheme begins in 2027.
Notwithstanding substantive draft texts prepared before the assembly, there is still plenty of negotiating to do before we know its final shape. And despite the pitfalls of carbon offsetting and some difficulty with integrating the scheme with the Paris process, a resolution at the meeting would be a step forward (to be followed by further steps and leaps) for an industry with emissions roughly equal to those of the entire nation of South Korea.
The authors will be attending the 39th ICAO Assembly in Montreal.