Ripple effects of climate change have created a 'new ocean'

The impact of climate change on marine ecosystems is becoming increasingly complex. A recent study offers a new perspective on how climate change and local temperature fluctuations interact to affect marine life.

Study co-author Jon Witman, a professor of Biology at Brown University, emphasized the importance of understanding both the steady warming trend and local temperature variability.

How organisms adapt

Witman collaborated with Andrew Pershing of Climate Central and John Bruno from the University of North Carolina at Chapel Hill. 

The researchers investigated how marine communities adapt to both gradual and variable changes, reviewing processes that either facilitate or hinder adaptation.

Studying climate and the ocean

“Greenhouse gas emissions are warming the ocean with profound consequences at all levels of organization, from organismal rates to ecosystem processes. The proximate driver is an interplay between anthropogenic warming (the trend) and natural fluctuations in local temperature,” wrote the study authors. 

“These two properties cause anomalously warm events such as marine heatwaves to occur with increasing frequency and magnitude.”

Temperature fluctuations 

However, the interaction between the steadily warming climate and the spikes in local temperatures tends to be underappreciated, said Witman.

“Climate change studies often focus on the trend of global warming. But organisms in the ocean are also experiencing temperature fluctuations, and that’s less studied and therefore less understood,” explained Witman. 

“What we’re trying to do is to add more reality into ocean climate change studies by considering both the smooth, upward trend of climate warming as well as the variability on top of that trend.”

Local conditions

“Marine communities are made up of species that are adapted to the prevailing local conditions. Thermal conditions can be characterized by the mean temperature, the amplitude of the annual cycle, and the interannual variability,” wrote the researchers.

“These characteristics of ocean temperature strongly influence countless aspects of the physiology and ecology of marine organisms, the composition and diversity of marine communities, and even the structure and dynamics of marine food webs.”

Abrupt heat spikes

The research highlights that while marine organisms like coral attempt to adapt to gradually rising temperatures, they are also subjected to abrupt spikes, like heat waves, which have dire consequences. 

These heat spikes can lead to consequences like coral bleaching, where stressed corals expel algae and turn white, potentially leading to death if conditions don’t improve. 

Witman pointed to the increased incidence of such events in regions like the Mediterranean, which have seen significant coral bleaching and death.

Temporal ocean climate variation 

“Aspects of temporal variation such as the frequency of repeated temperature anomalies and delays in ecological changes (legacy effects) following anomalies are central to understanding the different ecological impacts of climate change. These processes can either accelerate or slow the rate of change and adaptation,” wrote the researchers.

“There is growing evidence that sessile organisms like corals in more variable thermal habitats are more resistant to thermal anomalies via ecological hardening.”

Vulnerable ecosystems 

Extreme events such as heat waves may alter or damage marine ecosystems in ways that leave them more vulnerable to both progressive climate change as well as the next temperature fluctuation, Witman added. 

A more realistic model may help scientists better identify areas where coral is more likely to die off in an extreme event leaving coral-dependent organisms at risk over time, he said.

Variable events

Witman said the responses to variable events like heat waves compound (and are compounded by) the effects caused by rapidly and steadily increasing ocean temperatures.

The study highlights the necessity of considering both steady and variable changes in climate studies. 

“If we just study how organisms respond to the smooth trend, we miss all the variability, which is driving ecological change,” Witman said. “It’s not just a matter of worsening physiological stress over time; there are also variable events that have their own ripple effects.”

Smooth and spiky

“The purpose of this review is to advance our understanding of how marine ecosystems have changed and will change in response to climate change,” wrote the researchers.

“We are specifically interested in understanding the limitations of current approaches and identifying experimental, conceptual, and theoretical approaches necessary to understand the new ocean.”

“A key theme of our review is that climate change in the ocean is both smooth and spiky. By smooth, we mean that there are clear temperature trends driving change. For example, Harris et al. interpreted warming as a press perturbation. By spiky, we mean that there is natural, inherent variability on top of the trend.” 

“The smooth trends and spikes from variability interact, and understanding how processes within organisms and communities filter these interactions is the key to understanding how marine ecosystems will change over the next century.”

Regions of concern

A global model developed by the team pinpointed regions of particular concern in the Gulf of Maine, the Caribbean Sea and the Mediterranean Sea. In these areas, there are high probabilities of exceptional warming events and “ecological surprises,” wrote the study authors. 

“These areas, especially, warrant investigation to improve our understanding of what’s going to happen in the future – as well as our conception of what we’re calling, ‘the new ocean,’” said Witman.

Future modeling

“Published models of species range changes assume a smooth future of gradual change where species declines are due to the trend rather than infrequent extremes (variance), they do not consider geographic variability in temporal variability, and they assume that dispersal and range expansion are smooth (and largely unlimited),” wrote the study authors. 

“Future modeling efforts need to incorporate these aspects in order to make more realistic predictions.”

“Future marine climate change ecology should consider both the temperature trend and inherent temperature variability, how they interact to exceed thresholds producing extreme events, and how they vary regionally to affect individuals, populations, and communities.”

The study is published in the journal Annual Review of Ecology Evolution and Systematic. 

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