The sun skims low on the horizon as I snatch wakefully for the midnight alarm. Sleep is hard when it’s never dark. Unthreading safety straps, I roll out off the top bunk to the roiling floor, scratchy commercial carpet pitching on the wilds of the Southern Ocean. The stabilizer screeches like a demonic playground swing as it throws the ship back the opposite direction from the lurch of a wave. Foam laces the porthole, glinting over ink-slush seas.

That’s a memory from 2007, when I sailed on Australia’s now-retired icebreaker, the Aurora Australis, as part of a scientific research voyage monitoring oceans and biology around Antarctica. My role involved collecting water samples to monitor changes (potentially due to climate change) in water properties around the continent.

During the last deglaciation fifteen thousand years ago, global sea level rose nearly sixty feet in less than five hundred years. Known as meltwater pulse 1A, this event was discovered in 1989 via shallow-dwelling corals found in the deep sea, bolstering emerging theories that climate change could be rapid. Ice and sediment cores in Greenland had already revealed evidence of recurrent spikes in atmospheric temperature of 10 to 15 degrees Celsius during the last sixty thousand years, and these spikes were closely spaced in time with other sea-level rise events. Ice sheet formation and decay clearly played a role in these climate changes, and the game was on to determine how, and how much. As a newly minted scientist, I wanted to play.

The ice sheets over North America and Europe disappeared around ten thousand years ago, leaving just Greenland behind. Antarctica has been steadier. Cut off from the rest of the world thirty-four million years ago when the passage between the tip of South America and the Antarctic Peninsula opened at the same time global atmospheric CO2 levels were decreasing, Antarctica has become a runaway train of ice accumulation. The circumpolar ocean acts like a flywheel, flinging out much of the heat from further north that tries to penetrate south. While fossils from the Eocene period fifty million years ago reveal lush vegetation in Antarctica, ice sheets one to three miles thick now cover much of the continent, locking up water that would otherwise be in the ocean. Antarctica is the largest ice mass on the planet, containing enough water to raise global sea level by more than 160 feet. For comparison, Greenland has an ice mass equivalent to around a twenty-three-foot global sea-level rise.

Yet sea-level rise projections from the Intergovernmental Panel on Climate Change did not, until recently, include the effects of melting ice sheets. This is because the dynamics of ice sheet accumulation and decay are nonlinear and hard to quantify. So, while we understand many individual effects – for example, we know that ice melts when it is warmer, that warmth is driven by increases in greenhouse gases, and that heat takes time to reach Antarctica – we don’t fully understand how these different effects, or drivers, interact, at least not in exact, quantifiable ways. The effect of a small push from one driver can be largely unnoticeable. But if all the pushes line up in the same direction or pass a certain threshold, they can start to reinforce each other and become runaway. Prediction certainty from complex nonlinear systems is low.

Scientists like me are drawn to such intricate systems and, despite common perceptions from nonscientists, seeking to understand our world need not be a domineering or disenchanting pursuit. The act of witnessing, examining, and attempting to comprehend the vast complexity of the natural world often leads scientists to experiences of awe and wonder, congruent with religious experiences or what others feel in the presence of great art. Awe inspires. Its experience, writes Alister McGrath in The Territories of Human Reason, “creates a new receptivity towards increasing understanding, thus offering a powerful stimulus to the scientific engagement with nature.” This engagement need not demand complete and total knowledge. For Einstein, nature’s “magnificent structure” was something “we can only grasp imperfectly,” and most scientists would agree. We’re not all cold, calculating types. But we try to understand.

Orbital Cycles

Of course, continuing a decade of shiftwork, I got assigned to nightshift on the icebreaking research vessel. Except it wasn’t dark. The tilt of the Earth aims the poles toward the sun, so it’s always daytime there in summer. A particularly warm summer can result in more ice melt than normal. Less ice decreases reflectivity, allowing the region to absorb more heat, which melts more ice. Maybe, argues a leading theory, so much melt would occur that the next winter could not re-accumulate all the lost ice. The whole ice sheet could melt over a series of warm seasons. Alternatively, successive colder seasons could lead to snow cover persisting through summer to eventually become permanent ice sheets.

We know that summers and winters are periodically warmer or colder due to cyclical changes in the Earth’s orbit known as Milankovitch cycles. Eccentricity, where the Earth’s orbit alternately squashes and unsquashes like a wire ring, and obliquity, where the tilt of the Earth waxes and wanes, are dominant, enhancing or weakening seasons. Long-term geological records of ice and temperature trends clearly follow these cycles. Less clear is why climate history reflects these cycles more strongly or weakly at different times and why one is sometimes more dominant than another.

Carbon Cycles

Cycles surround us. Overlaid on the push of orbital cycles are the pushes of atmospheric greenhouse gas concentrations. CO2, a major greenhouse gas, cycles through animals, plants, the Earth’s crust, and oceans over the millennia, becoming trapped as tiny air bubbles in ice and leaving markers in sediment from which climate scientists can take core samples. CO2 blankets the Earth to varying degrees, preventing the day-to-night temperature swings of over 100 degrees Celsius that happen on Mars.

Oceans, in particular, absorb great masses of CO2 and transport it downward in deep vertical currents to the “carbon sink,” preventing, at least initially, atmospheric CO2 from rising as high as it would otherwise. Here, CO2 is deposited and buried on the ocean bed. Or CO2 may outgas from the ocean when the water returns to the surface hundreds to thousands of years later.

The marine surveyors send down a camera. We watch crinoids in real-time bounce like orange long-haired wigs, breathing in oxygen that the ocean currents have taken down.

The Earth breathes.

Ocean Cycles

And the icy poles are the lungs. Ocean ventilation cycles are directly driven by the icy polar regions since that is where water gets coldest. Cold water is denser than warm water. Salt water is denser than freshwater. Cold, salty water is densest. If sufficiently dense, it sinks – kind of like the opposite of “warm air rises.” It is convection in reverse but in the oceans not the air. Salinity is key, as it affects water density more strongly than temperature. Without sufficient salinity, water can’t cool below the temperature required to give it the density to sink, and our oceans stagnate.

In Antarctica, the source of salinity is freezing sea-ice. Since freshwater preferentially freezes over saltwater at the same temperature, the formation of abundant sea-ice like that which we were breaking through each day (always I thought of Shackleton’s ship, stuck fast till the ice squeezed it to crumpling point) leaves behind water that is saltier than the original. Then, this salty water left behind by forming sea-ice sinks. Deep under the Weddell and Ross Seas, it waterfalls off the continental shelf to the abyssal plain four miles below and spreads north, one part of the global ocean ventilation. Antarctic Bottom Water (AABW) is the deepest deepwater in the world and it’s why we were here: we wanted to see if its composition and formation rate were indicative of healthy breath.

Flickering screens line the CTD (Conductivity, Temperature, Depth) control room and a computer bank sits in the middle. At 1:00 a.m. the CTD night shift leader, Mark, professionally explains the intricacies of sample collection. Thus prepared, we don oilskin overalls and boots and thwump to the wet room to prepare the CTD rosette, which looks like a bunch of gas cylinders arranged on a circular rig. A giant winch hook punctures the top middle. Once prepared, the rig is secured to a chain and the ship’s crew takes over, craning it out from the ship’s side, then lowering it through heaving swell.

Back in the control room, a walkie-talkie code of “slow,” “hold,” “stop” guides the rosette down to gently bump bottom. Then it’s pulled up in stages, stopping at predetermined levels to tap a key and pop off a cylinder top that lets water in. When it’s back on board, we quickly extract samples of water. Erlenmeyer flasks full and hands numb – hurry, the onboard hydrochemist wants to analyze the samples immediately – pinpoints of data to which we try to fit nets of understanding.

There’s a lot of data collection and categorizing before glamorous theorizing.

Ventilating deepwater also forms in the Northern Hemisphere, specifically in the North Atlantic, where it is known as North Atlantic Deepwater (NADW). Here, the essential salinity is supplied via the Gulf Stream and North Atlantic currents from near-tropical areas further south. These ventilating currents in the Northern and Southern Hemispheres connect in the depths: a great conveyer of water, salt, dissolved gases, suspended particles, and heat. Together, AABW and NADW complete a circuit of ocean ventilation – the ocean breathes.

And it can stop.

Oceans as Climate Modulator or Amplifier

In the late 1980s, scientists examining sediment cores from the ocean floor noticed patterns that indicated NADW had been much weaker in the past. Now, it is widely accepted that there was at least one NADW shutdown during the last glacial period. This backed up a theoretical model that an oceanographer named Henry Stommel had advanced in the early 1960s.

Stommel hypothesized that when the source of salinity is remote (way back in the equatorial Atlantic), it is possible to create a feedback loop: freshen North Atlantic surface water (by melting a nearby ice sheet, say) and it becomes less dense, so deepwater sinking slows. Once sinking slows, less warm, salty surface water is drawn northward, which leads to further freshening in the North Atlantic. Sinking then slows further, and the slowdown becomes self-feeding. With sufficient freshwater, the overall circulation system (known as Atlantic Meridional Overturning Circulation, or AMOC) can come to a near standstill.

The North Atlantic currents are what moderate the climate of lands around the North Atlantic. Western Europe, in particular, and the northeastern United States have a much milder climate than Siberia or inland Canada thanks to the Gulf Stream and North Atlantic currents. A stalled AMOC disrupts global heat distribution. Temperatures over Greenland could drop as much as 10 degrees Celsius, with lesser drops of an estimated 2 to 5 degrees in western Europe and near the northeastern United States, strongly dependent on the accompanying atmospheric CO2 concentration. Areas to the south would become much warmer, the contrast likely driving unprecedented weather events. Note that these are annual average temperatures; extremes, which are more noticeable and have a greater effect on life, would change even more. Many climate scientists believe there is a non-negligible risk of an AMOC shutdown happening over the coming century.

Why Bother Monitoring?

The AMOC is considered so vital that it has been continuously monitored since 2004. Hundreds of sensors are strung like lights across the ocean to record temperature, salinity, and other parameters indicative of strength. There are signs of a decrease.

It’s fair to question the point of such monitoring. I often wonder myself. What do we do if we see signs of an imminent collapse?

In meteorology, observations allow us to forecast extreme weather events so people can take action. But in longer-term climate modeling, it’s unlikely we’ll do anything in response to a dire forecast, at least given our track record in combating climate change so far. Are we demanding to know everything before we do anything? Are we forgetting to be awestruck by what we already know?

Antarctic Bottom Water is also monitored, though not continuously. Back in 2007, I was researching how stable AABW was, to anticipate how much it might be affected by possible human-induced climate changes. It’s less susceptible than NADW. Persistent winds tend to push surface freshwater away north, sea-ice removes surface freshwater as it freezes, and sufficiently salty water still manages to sink. It is not completely invulnerable, though. Observations from repeat samplings over recent decades show warming, freshening, and reduced formation rates. The combined effects of atmospheric warming and warm water encroaching south are destabilizing ice sheets.

The ice sheet in West Antarctica – the part with the elbow-like peninsula – is most vulnerable to collapse because a lot of it is not grounded; it fans out between underlying rocky peninsulas, linking across large tracts of ocean. The East Antarctic Ice Sheet is more stable but also reaches out into the ocean in vast ice shelves, where it is vulnerable to basal melting from warming oceans. In 2002, the Larsen B ice shelf – the size of Long Island – shocked scientists by collapsing into the ocean over just a few weeks. In 2022, the Conger ice shelf – a bit smaller than Rome – collapsed. The Greenland ice sheet is also showing clear signs of weakening.

The Earth’s climate changed drastically in the prehuman past as well. Cycles have been plowing on for millennia. But it’s also clearly a finely balanced system. So, while it’s true there is high uncertainty around the exact response of ice sheets and ocean currents to changing climate, there is high certainty that they do respond.

The demand for more certainty around the exact impacts of climate change before taking action is a common refrain. I’m less sympathetic than I was; certainty is a mirage, and it’s worth remembering we don’t have perfect predictive power or understanding in other fields either. Sometimes, the public seems more demanding of certainty than the scientists.

Will We Change?

I am tired of being asked to justify my “opinion” on climate change. None of us is completely impartial, but evidence is not opinion. Even in meteorology, where I have worked for the last fifteen years, I can’t avoid climate questions and find myself making more frequent “unprecedented” forecasts compared to past years.

So, knowing what I do, do I live as I know one should? I’m too small to make a difference to climate change, after all, probably, right?

But what we do at home seeps into the world at large, whether it’s our energy usage and carbon footprint, or acts and attitudes of hypocrisy and nihilism.

We don’t get to jettison personal responsibility just because governments and corporations let us down.

In Australia, we live in houses bigger than ever before, which require more energy to heat and cool and rarely consider passive solar design, which allows warming sun inside in winter but blocks it in summer. Air-conditioning use has skyrocketed – so much for the Australian reputation of being good with the heat. In the Northern Hemisphere, meanwhile, in winter we are heating homes so far above ambient that we barely need long sleeves. The International Energy Agency found that turning the thermostat down just one degree could cut European gas demand by 7 percent.

Living in a way that is sustainable, rather than stripping our world to within an inch of what we can get away with, is an act of creation care. It is also an act of loving our neighbor: those living on low-lying islands or in overcrowded cities who can’t afford air conditioning, heating, or flood insurance will be the most immediately affected by climate change. It is also the appropriate response to awareness of our smallness in the face of vastness, an acceptance of the fact that the world is not ours to consume, control, or even fully comprehend. For scientists, this calls for scientific humility that counters Enlightenment promises of one day understanding everything and hence, in a sense, bringing the world to heel. For Christians, it is one more call to hold together the knowledge that we are never fully in control with the command to act nevertheless. We should not despise “the day of small things” (Zech. 4:10) in which we live, nor languish in inaction waiting for God to do big things.

In January this year, European scientists extracted the longest continuous ice core to date, the bottom portion containing trapped air bubbles and particles that are over one million years old and possibly up to two million years old. Analysis will reveal some climate secrets and leave us in awe of the complexity of our world, but others will remain secret. The scientists and the rest of us will go home, and we will all have to keep living as best we can as the planet moves, the CO2 cycles turn, the oceans breathe, and the ice creaks and heaves in response.