The oceans are heating up as the planet warms.

This past year, 2024, was the warmest ever measured for the global ocean, following a record-breaking 2023. In fact, every decade since 1984, when satellite recordkeeping of ocean temperatures started, has been warmer than the previous one.

A warmer ocean means increased evaporation, which in turn results in heavier rains in some areas and droughts in others. It can power hurricanes and downpours. It can also harm the health of coastal marine areas and sea life – coral reefs suffered their most extensive bleaching event on record in 2024, with damage in many parts of the world.

Warming ocean water also affects temperatures on land by changing weather patterns. The EU’s Copernicus Climate Change Service announced on Jan. 10 that data showed 2024 had also broken the record for the warmest year globally, with global temperatures about 2.9 degrees Fahrenheit (1.6 Celsius) above pre-industrial times. That would mark the first full calendar year with average warming above 1.5 C, a level countries had agreed to try to avoid passing long-term.

Climate change, by and large, takes the blame. Greenhouse gases released into the atmosphere trap heat, and about 90% of the excess heat caused by emissions from burning fossil fuels and other human activities is absorbed by the ocean.

But while it’s clear that the ocean has been warming for quite some time, its temperatures over the past two years have been far above the previous decades. That leaves two mysteries for scientists.

It’s not just El Niño

The cyclic climate pattern of the El Niño Southern Oscillation can explain part of the warmth over the past two years.

During El Niño periods, warm waters that usually accumulate in the western equatorial Pacific Ocean move eastward toward the coastlines of Peru and Chile, leaving the Earth slightly warmer overall. The latest El Niño began in 2023 and caused global average temperatures to rise well into early 2024.

But the oceans have been even warmer than scientists expected. For example, global temperatures in 2023-2024 followed a similar growth and decline pattern across the seasons as the previous El Niño event, in 2015-2016, but they were about 0.36 degrees Fahrenheit (0.2 Celsius) higher at all times in 2023-2024.

Scientists are puzzled and left with two problems to solve. They must figure out whether something else contributed to the unexpected warming and whether the past two years have been a sign of a sudden acceleration in global warming.

The role of aerosols

An intriguing idea, tested using climate models, is that a swift reduction in aerosols over the past decade may be one of the culprits.

Aerosols are solid and liquid particles emitted by human and natural sources into the atmosphere. Some of them have been shown to partially counteract the impact of greenhouse gases by reflecting solar radiation back into space. However, they also are responsible for poor air quality and air pollution.

Many of these particles with cooling properties are generated in the process of burning fossil fuels. For example, sulfur aerosols are emitted by ship engines and power plants. In 2020, the shipping industry implemented a nearly 80% cut in sulfur emissions, and many companies shifted to low-sulfur fuels. But the larger impact has come from power plants reducing their emissions, including a big shift in this direction in China. So, while technologies have cut these harmful emissions, that means a brake slowing the pace of warming is weakened.

Is this a warming surge?

The second puzzle is whether the planet is seeing a warming surge or not.

Temperatures are clearly rising, but the past two years have not been warm enough to support the notion that we may be seeing an acceleration in the rate of global warming.

Analysis of four temperature datasets covering the 1850-2023 period has shown that the rate of warming has not shown a significant change since around the 1970s. The same authors, however, noted that only a rate increase of at least 55% – about half a degree Celsius and nearly a full degree Fahrenheit over one year – would make the warming acceleration detectable in a statistical sense.

From a statistical standpoint, then, scientists cannot exclude the possibility that the 2023-2024 record ocean warming resulted simply from the “usual” warming trend that humans have set the planet on for the past 50 years. A very strong El Niño contributed some natural variability.

From a practical standpoint, however, the extraordinary impacts the planet has witnessed – including extreme weather, heat waves, wildfires, coral bleaching and ecosystem destruction – point to a need to swiftly reduce carbon dioxide emissions to limit ocean warming, regardless of whether this is a continuation of an ongoing trend or an acceleration.

This article has been updated with Copernicus Climate Change Service’s global 2024 temperature data.

Annalisa Bracco, Professor of Ocean and Climate Dynamics, Georgia Institute of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

What does the future hold for forests in a warmer, drier world? Over the past 25 years, trees have been dying due to effects of climate change around the world. In Africa, Asia, North America, South America and Europe, drought stress amplified by heat is killing trees that have survived for centuries.

Old trees may have grown through entire millennia that were wetter than the past 20 years. We are scientists who study forest dynamics, plant ecology and plant physiology. In a recent study, we found that trees can remember times when water was plentiful and that this memory continues to shape their growth for many years after wet phases end.

This research makes us optimistic that young trees of today, which have never known 20th-century rainfall, have not shaped their structure around water abundance and thus may be better equipped to survive in a chronically dry world.

What if we water the forest?

This study built on nearly 20 years of forest research in response to early warning signs of forest loss in the 1990s in the dry Rhône River Valley of the Swiss Alps. At that time, scientists observed that Scots pine trees that had stood for around 100 years were declining and dying. They wondered whether drought or other climate factors were driving this loss.

To tackle this question, researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research designed an ecological experiment. To understand the impacts of drought, they would irrigate a mature forest, doubling natural summertime rainfall, and then compare how these water-rich trees fared in comparison with those receiving only natural precipitation.

The Pfynwald experiment, launched in 2003, has shown that trees survived at higher rates in irrigated plots. After 17 years of irrigation, the team found that irrigation didn’t just help trees survive dry phases – it also increased their growth rates.

Legacy effects are forests’ memories

Trees experiencing drought alter their leaves, wood and roots in ways that prime them for continued dry conditions. Wood under drought might have smaller cells that are less vulnerable to future damage, and roots might increase relative to leaf area. These structural changes persist after the drought has passed and continue to influence the tree’s growth and ability to tolerate stress for many years.

Known as “legacy effects,” these lingering post-drought impacts represent an ecological memory of past climatic conditions at the tree and forest level. Knowing that trees hold a persistent memory of past dry phases, researchers wondered whether they might also show structural changes in response to past wet periods.

Eleven years after summertime irrigation started in Pfynwald, scientists stopped irrigating half of each plot in 2013 to address this question. The formerly irrigated trees, which at this point were about 120 years old, had experienced a lasting period of irrigation – but now those times of plenty were over.

Would the trees remember? A decade later, we found out.

Trees, trains and particle accelerators

On an early March morning in 2023, two of us (Alana Chin and Marcus Schaub) met at Pfynwald to collect very fresh leaf and twig samples so that we and colleagues could look inside to search for signs of lasting effects of past water richness.

At the site, we climbed canopy access towers to collect newly grown treetop leaves and twigs from control trees that had never been irrigated; trees that had been irrigated every summer since 2003; and formerly irrigated trees that had not received irrigation water since 2013.

We took our samples to the Swiss Light Source, an intensely powerful synchrotron – a type of particle accelerator that produces the world’s most intense beams of light. This facility is the home of the TOMCAT, an extremely high-resolution X-ray that allowed us to look inside our leaves and twigs without disturbing their structure.

Scanning our samples took all night, but when we stumbled out of the building, we had images capturing every cell in exquisite detail.

The memory of water

We found that the new leaves of once-irrigated trees were different from both continually watered trees and never-watered control trees. Leaves carry out photosynthesis that fuels a tree’s survival and growth. Inside them, we could see the legacy of past water abundance, written in the size, shape and arrangement of cells.

Reading this cellular signature, we observed that, at the expense of structures promoting productivity, formerly irrigated trees showed every sign of chronic water stress – even more so than never-irrigated trees. In their anatomy, we saw why these trees that had it easy for 11 wet years were now growing slowly.

Every cell in a leaf comes with a trade-off. Trees must balance investments in rapid photosynthesis with others that promote leaf survival. Rather than building the cells used to harvest sunlight and ship sugar to the rest of the tree, leaves on the trees that had been irrigated showed every indication of drought stress we could think to measure.

After receiving extra water for an 11-year stretch and then losing it, the trees were producing new, tiny leaves that invested mostly in their own survival. The leaves were structured to protect themselves from insects and drought and to store water reserves. Compared with leaves on trees that had never known irrigation, these looked as though they were in the middle of the drought of the century.

While this memory of water might seem negative, it likely once helped trees “learn” from past conditions to survive in variable environments. The formerly irrigated trees did not know that humans had played a trick on them. Like trees experiencing climate change, they had no way of knowing that the water was not coming back.

When trees experience a drought event, recovery can mean reaching a “new normal” state, in which they are prepared to survive the next drought, with smaller, less vulnerable cells and increased energy reserves to ‘save up’ for future dry periods. They may have deeper roots or a smaller pool of leaves to support, helping them prepare for an unstable environment.

We wanted to know whether the same was true of trees that had experienced water abundance. Were they waiting in distress for the water to return?

Hard times may make tough trees

In some temperate forests, like the ones we studied in Switzerland, old trees once knew levels of water abundance that now are gone, thanks to climate change. That past abundance may have locked into place structural and epigenetic changes in the trees that are mismatched to today’s drier world. If this is true, then some of today’s devastating global tree mortality events may be, in part, due to the legacy effects of past water abundance.

In most of the world’s temperate forests, however, the current cohort of young forest trees – those sprouting in the past 15 to 20 years – has managed to establish itself under conditions that once would have been considered chronic drought. Those young trees, which have survived an endless dry period, will form the forests of the future.

In all, our observations in Pfynwald have provided us some room for hope that young trees currently taking their place in many forests worldwide may be better prepared to cope with the world as humans have shaped it. Climate shifts in recent decades have primed them for hard times, without the lingering memory of water.

Alana Chin, Assistant Professor of Plant Physiology, Cal Poly Humboldt ; Janneke Hille Ris Lambers, Professor of Environmental Systems Science, Swiss Federal Institute of Technology Zurich, and Marcus Schaub, Group Leader, Forest Dynamics and Ecophysiology, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL)

This article is republished from The Conversation under a Creative Commons license. Read the original article.