Anyone concerned about human-induced planetary warming will know of the mounting damage and disruption directly linked to rising temperatures. Sea levels have risen significantly, triggering more frequent flooding in coastal areas, especially in coincidence with hurricanes. More frequent and prolonged droughts have affected some regions, including the US Southwest. Episodes of extreme temperatures have grown more frequent, triggering deadly heatwaves and stoking wildfires.

For the most part, these are the expected direct effects of warming. But it’s fair to wonder if we’re mostly focused on these direct effects only because they’re the easiest to see and comprehend. On a planet far away from any kind of thermodynamic equilibrium, dominated by rich geophysical and biological feedbacks, the most serious consequences of warming may be indirect. Many are only now beginning to come into view.

As one example, scientists are closely monitoring ongoing and accelerating shifts in the timings of key biological events — the springtime flowering of trees, nest-building and hatching of eggs, changing colours of leaves in the autumn. Early evidence suggests many aren’t changing rapidly enough to keep pace with the speed of current warming. There’s also the potential for such changes to disrupt the delicate biological coordination of the activities of millions of interacting and interdependent species on which the world’s ecosystems depend. These effects are far more subtle and unpredictable than the direct consequences of a warmer Earth, but may turn out to be far more important.

Many of us see global warming as something that is difficult to detect, evident only in sophisticated estimates of globally averaged sea-surface temperatures, atmospheric CO2 concentrations, or the fine statistical patterns of extreme atmospheric events. An entire science of attribution now focusses on identifying individual events that can be linked to warming in a statistically significant way. Yet warming is also evident in other changes taking place quite literally all around us.

Phenology is the technical term for the study of the timing of key events in biological life cycles, such as flowering, first flights, egg-laying and migrations. In Kyoto, Japan, records on the first blossom of cherry trees go all the way back to the year 812. This data, and a variety of other records from around the world (Y. Vitesse et al., Nat. Clim. Change 12, 300–302; 2022) indicate that plants’ flowering and first-leaf dates changed little through the nineteenth century, then started to advance between 1900 and 1950 following rising planetary temperatures. Phenological shifts have accelerated since then, advancing by as much as 30 days in some places. In 2021, the cherries in Kyoto bloomed on the earliest date seen for 1,200 years.

It’s not surprising, of course, that nature is responding to rising temperatures — species need to adapt, changing their behaviour to fit new conditions. Are they adapting fast enough? Worryingly, some studies suggest that many are not.

One recent study (Y. Song et al., AGU Adv. 2, e2021AV000431; 2021) looked at the pace of phenological changes for plant species in mid- and high-latitude regions of the Northern Hemisphere, comparing the observed shifts to local changes in temperature over the past 30 years. The study used remote sensing to look at many species together, recording changes in the dates of greening in springtime and plant senescence in autumn. The study found a significant mismatch between the plants’ responses and patterns of temperature change, with most plants lagging behind the pace of recent warming, some even shifting their timings in the wrong direction. The study also found that these mismatches were more pronounced in landscapes degraded by farming and other human activities.

It’s not entirely clear what is causing this failure to respond, but how plants respond to changing temperatures isn’t simple. It may seem obvious that higher temperatures would always cause earlier growth, but some plants need sufficient chilling in winter to prepare for growth, and higher temperatures can spoil their preparation. There’s also another possibility, as species respond not only directly to temperatures but also to the shifting activities of other species on which they depend. Hence, species in undisturbed regions may be responding more quickly because they’re exposed to more signals about the changing environment. In contrast, the slower responding plants in zones degraded by human impacts may be lagging behind as they lack access to the normal signals that would alert them to changes taking place.

Phenology isn’t only about time, but also space, as both plants and animals can adapt to a changing environment by changing their location, though plants obviously do so more slowly than animals. This is another related aspect to nature’s indirect responses to our changing climate, and one that should cause us considerable concern. Ecologists are also seeing abundant evidence that many species are gradually drifting further toward the poles as the planet warms. One consequence of this (C. J. Carslon et al., Nature; 2022) is the likely mixing together in habitats of species that have never encountered one another before.

Of course, migrating species carry their parasites and pathogens along with them, creating myriad opportunities for pathogens to hop from one host to another, finding new reservoirs for further transmission and evolution. With an estimated 10,000 distinct viruses capable of infecting humans currently circulating in wild animal populations, we can expect the natural migrations of species in response to rising temperatures to create conditions conducive to many future pandemics, some on the scale of the COVID-19 pandemic. Carlson et al. projected likely geographic range shifts for some 3,139 mammal species in response to warming, and in combination with expected changes in land use. Their analysis foresees more than 4,000 likely cross-species transmissions of viruses, with many of these having a good chance to make it into humans.

Planetary warming is of course going to cause things like more frequent and serious floods, fires and droughts, sea-level rise, and so on. But seeing such direct consequences of higher temperatures as the most problematic aspects of warming may only reflect our collective lack of imagination. Far more important may be the indirect consequences as the entire ecosystem that surrounds and supports us begins to shift unpredictably.