Climate change is messing with the seasons. Here's why that matters.
A celebration of Earth’s seasons on Earth Day — and the risks we face as they unravel
This week felt like the right moment to revisit this topic. Daniel Hernández-Carrasco, a PhD student in my lab just became New Zealand's Frontiers Planet Prize National Champion1 for a paper (summarised here, and in depth here2) that sits right at the heart of what I'm about to describe. Earth Day, a student prize, and a holiday spent watching ecosystems do their thing — sometimes the timing works out.
My kids started school again yesterday after a two-week holiday. We spent part of that time on New Zealand’s West Coast. I adore spending time there — the forests are lush, the rivers full of life, the coast absolutely wild.
Despite the whole family either still unwell or recovering from a cold, we spent almost all hours in the wild — getting lost in dense podocarp forest, fishing the wild rivers and estuaries, and toasting marshmallows on the rugged beaches.
These are the memories that make a childhood.
But why go there during these challenging financial times and even more in the middle of April given the notorious West Coast weather?
Well, that’s because the school holidays are set by the seasons.
School terms, public holidays, cleaning regimes, sports seasons, financial quarters, agricultural calendars, fishing and hunting seasons. Heck even planning a wedding. These are all things that are set by the seasons.
We often take seasonality for granted. Probably because it’s such an integral and implicit part of our lives.
For natural ecosystems, we’ve tended to overlook or undervalue this fundamental cycle. Yet, natural rhythms aren’t just background noise for ecosystems, they’re the foundation.
The natural world runs on the same clock
Go anywhere in the world and you’ll see that natural ecological systems run on the same clock we do. Species need it to know when to move, when to flower, and when to mate.
Take the stoat featured in the header image. In polar regions, they shift their coat colour to match seasonal changes in snow cover by tracking changes in day length through the year. They do this to help them blend in with their surroundings in the highly seasonal environments of those landscapes.
And they’re not alone — over 20 species of birds and mammals across the northern hemisphere go through a complete transformation every year, twice a year to match their environment. The primary benefit is camouflage — for both hunting and hiding from hunters.
In the American West, cottonwood gallery forests create oases of biodiversity along rivers that dissect harsh desert landscapes. These forests support hundreds of species — from migrating songbirds to deer, beavers, and native fish — concentrating life along narrow ribbons of water in an otherwise dry world.
Cottonwoods, like most of the species occupying these rivers, live by a very rhythmic clock.
The main driver? The melting of the snows.
We tend to think of river floods as being sources of devastation. But for the natural world, they bring life. When the rivers burst their banks, reproduction is possible for cottonwood. But, as I discussed a couple weeks ago, a river overtopping its banks is just one of the intricate criteria for cottonwood.
the flood has to fall within the window in which cottonwood release their seeds, it has to be large enough to inundate the floodplain, and the water has to recede at the right rate — too fast and the roots of the seedlings can’t keep up with the receding moisture, too slow and they sit in water.
So both timing and magnitude of the spring floods matter for the chances of cottonwood reproduction.
But why evolve to have such a tight relationship with the seasons you might ask?
Well, that’s because these rhythms have been so predictable for thousands of years. Precipitation in the Colorado River catchment tends to fall as snow most of the time. So it gets locked up in the hills and released when those hills warm up in spring.
Like the stoat, these trees expect the seasonal rhythm because their species has evolved with it for thousands of years. Seeds are released in synchrony with the meltwater, travelling to new places, and starting the cycle all over again.
There are countless other examples of such intricate connections with the seasons, whether that’s the intimate link between a plant’s flowering time and its pollinators, or the precise timing of animal migrations that track seasonal pulses of food, all the way through to the regulation of the climate system itself.
Unfortunately, many of these rhythms are no longer what they once were, and that’s putting biodiversity at risk the world over.
The rhythm is breaking
Colorado in the US has just come through a winter of record low snowpack. The melt is coming — in fact, it’s already happening — but there won't be enough of it to generate the floods these ecosystems depend on.
When these rhythms break once, species can cope. It’s when they continue to break over several years that the proverbial hits the fan.
As Bill McKibben recounted the effects of this record warm winter in the American West are widespread:
“I flew over peaks where I’ve glissaded down snowfields in June and there was not an inch of snow to be seen. Lake Mead from above looked like a bathtub with the plug open.”
And it’s not just in the US. Floods in Europe are arriving up to two weeks earlier or later compared to 50 years ago. And dams have already dramatically altered the timing of flood rhythms with dire ecological consequences.
These timing differences are critical. Beyond floods, species use cues like temperature and moisture to set their clocks. For instance, the cottonwoods I refer to require a certain amount of chilling to accumulate during the winter to break dormancy and set the stage for reproduction. Once that threshold is met, spring warming drives the process that lead to seed release. So warmer winters may delay this process resulting in a mismatch between spring floods and seed release.
In other words, the stars have to align for cottonwood regeneration to succeed: sufficient winter chilling, enough snowpack to generate spring floods, and perfectly timed spring warming for seeds to be transported onto moist soils.
Miss any one of these cues, and reproduction fails for that year.
To further complicate things, some species work off day length instead of environmental cues like temperature. For instance, whether it’s the stoat I referred to earlier, or Arctic foxes or snowshoe hares, day length tends to be the main cue for moult timing.
When the seasons shift, day length does not — with obvious consequences.
A brown rabbit in a snowy landscape is a pretty easy target and a white fox in a green landscape is pretty poor at ambushing prey. Unfortunately, such species are at risk as the only path for many of them to track climate change is through evolution.
Why this is a hard fix
Although biologists have studied seasonal processes for centuries, we know surprisingly little about how they mediate any ecological impacts of altered seasonality. And we are likely underestimating these impacts. That’s because the effects of these changes — including changes to the magnitude, timing, and predictability of seasonal events — propagate from the genetic structure of single populations up to the performance of whole ecosystems, such as carbon sequestration.
A small mismatch between moult timing in snowshoe hares could be the difference between survival and predation for an individual. But scale that up across a population, and you’re altering predator-prey dynamics. Alter those dynamics across a community, and you’re reshaping food web structure. Reshape enough food webs, and you’re affecting the very capacity of ecosystems to sequester carbon, retain nutrients, and sustain the services we depend on.
This is what makes changing seasonality so difficult to forecast. The effects don’t stay where they start. They travel upward through levels of biological organisation — from the individual, to the population, to the community, to the ecosystem — compounding and interacting as they go, often in ways that are impossible to predict by looking at any single level alone.
But there are things we can do. Where possible (e.g. rivers), preserving natural rhythms allow these critical moments for nature. Restoring natural rhythms can help restore ecosystems and conserve native species by helping them get a leg up over nonnatives. In other systems, like seasonal fisheries, harvests could be guided by in-depth monitoring and forecasts to avoid mismatches between target species and their prey.
Regardless of the system, understanding these intricate connections is imperative for improving the way we manage and predict biodiversity in a world of changing rhythms.
Conclusions
Japan’s famous cherry blossom season generates in the order of $9 billion annually through tourism. The flowering of these trees has been so regular that festivals, school calendars, and family gatherings have been built around their expected arrival.
But over the past century, the peak flowering time has shifted forward by around two weeks. What does that mean for Japanese society? Initially, it means a rescheduling of festivals. But these shifts can be disruptive and over time the disruptions can amplify. Some researchers have even proposed replacing the dominant species with more resilient varieties.
At the end of the day, we can shift our behaviour — by changing the timing of crop harvests or Indigenous fire-burning practices, for instance. But despite their remarkable resilience, species are often less capable if these rhythms are hardwired into their core machinery.
My family’s visits to the West Coast aren’t going to be destroyed by a shift in the timing of the school calendar. But a small mismatch between the timing of a flower opening and the life cycle of a butterfly can result in a complete failure to reproduce, which can ripple out and disrupt whole landscapes.
On this Earth Day, when we take stock of what climate change is doing to the planet, this is something most headlines won’t mention. Let’s not forget the very rhythms the world is run by. And spare a thought for the species that live by very strict biological clocks.
Time for me to pull out my winter wetsuit — the waves are calling and it’s cold here! I can adapt by adding a layer. Many species cannot.
In the meantime, let’s continue to share these messages loud and clear. Now is the time to act, not tomorrow.
Note, this in-depth post is for paid subscribers:
And the free summary:
For a more poetic story of the seasons:
Thank you, this is such an important post. Those seasonal shifts are creeping up on us, with their subtle changes year by year - although we start to notice now. And jet they can have big impacts which are difficult to bring to the attention, and like you say they go further than the shifts of our holidays.
Congratulations to your student, Daniel, for being New Zealand's Frontiers Planet Prize Champion. He is obviously doing great work. Isn't it a pity we don't hear more about this kind of work in mainstream media.
Seasonality is such an interesting subject and I really loved the Cottonwood example. So delicate is the balance for species living by a rhythmic clock. It really highlights the importance of seasonality research.