New research provides evidence of human ‘footprint’ on climate change

Claims that climate change is occurring naturally are contradicted by recent trends that point to human influence on ocean temperatures. A new ocean study provides clear evidence of human influence on climate change, showing that some human-induced signals are altering the amplitude of the seasonal cycle of sea surface temperature (SST).

“This is groundbreaking evidence that ocean temperatures are linked to human-caused climate change associated with increased CO2 emissions,” said co-author Benjamin Santer, a distinguished scientist and research assistant professor in the Division of Physical Oceanography at Woods Hole Oceanographic Institution (WHOI). ).

“We show that the anthropogenic signal in the seasonal cycle of sea surface temperature (SST) arises from the noise of natural variability. Geographic patterns of changes in the amplitude of the SST seasonal cycle (SSTAC) reveal two distinctive features: an increase in the mid-latitudes of the Northern Hemisphere associated with changes in the depth of mixed layers.” and a strong dipole pattern between 40°S and 55°S, mainly due to changes in the surface wind.” According to a new study published in the journal Nature Climate Change.

“The evidence we found is very clear. Our study is based on four different datasets of sea and ocean surface temperature observations. We analyzed data from a variety of monitoring systems, including satellite records and ocean measurements that WHOI has collected from ships since 1950. All this “The data revealed the same story and the same conclusion: The anthropogenic signal in SSTAC is very strong and has a very different structure,” said co-author Dr. Jia-Zhui Shi.

Evidence and inferences

The pattern of SSTAC change predicted by the model can be identified with high statistical confidence in four different observed SST products and 51 individual realizations of the historical climate evolution model. Modeling incorporating historical changes in individual forcing shows that increasing greenhouse gas emissions are the main driver of changes in SSTAC, with smaller but distinct contributions from anthropogenic forcing of aerosols and ozone.

The study was inspired by previous work by Santer, who has been working on climate footprints for more than 30 years. Previous studies have used satellite records to identify human fingerprints in the changing seasonal cycle of mid- and upper-tropospheric temperatures. But this is the first fingerprint study to reveal detailed patterns of climate change in seasonal sea surface temperatures.

Climate change and its impact on the ocean

“ The amplitude of the seasonal cycle of sea surface temperature changes and increases. One of our biggest findings is that there is more warming in summer than in winter. In both the northern and southern hemispheres, mixed layers of the deep ocean are thinning, which can significantly increase summer temperatures,” Shi said. “In the northern hemisphere, warming is more severe, associated with smaller ocean basins. “We found that in the Southern Hemisphere, changes in sea surface temperature are driven primarily by changing wind patterns caused by atmospheric warming.”

“This study refutes claims that recent temperature changes, whether due to the Sun or internal cycles in the climate system, are natural. A natural explanation is nearly impossible, given what we see here: changes in seasonal ocean temperatures.” said Santer. “This study also refutes the claim that we don’t need to take climate change seriously because it’s natural.”

Social and environmental consequences

“This strong human fingerprint on the seasonal cycle of ocean surface temperature is expected to have far-reaching impacts on marine ecosystems. This can significantly impact fisheries and nutrient distribution,” Shi said. “Understanding anthropogenic impacts on seasonality has scientific, economic and societal importance.”

In 2023, the heat content in the upper ocean reached the highest level recorded, causing serious concern in the scientific community. The ocean absorbs approximately 90 percent of Earth’s excess heat from global warming and plays a vital role in regulating planetary climate systems.

“The temperature of the ocean is literally rising. A lot of people want to know what’s going on,” Santer said. “A big part of the answer is that human activity is gradually warming the world’s oceans. The scientific community has focused on changes in average annual ocean temperature. This paper shows that fingerprinting along with seasonal changes is extremely important,” he said.

The ocean is a vital carbon sink, absorbing 25 percent of the carbon dioxide we produce by burning fossil fuels. But the ocean’s ability to absorb CO2 ₂ depends on temperature. As the ocean warms, it is critical to understand how this affects the oceans’ ability to absorb CO2_2.

“As oceans absorb carbon dioxide, it leads to acidification, which can negatively impact marine organisms. If we start changing the pH of the ocean, we run the risk of affecting the structural integrity of organisms at the base of the food chain,” Shi said.

“We now face important decisions about what we should do about climate change in the United States and around the world. These decisions should be based on our best scientific understanding of the reality and severity of human impact on average climate and seasons,” Santer said.