A new study affirms the global warming hiatus, or the global warming slowdown, experienced from 1998 to 2013 was caused by the oceans absorbing more and more heat, and isn't proof global warming isn't man-made or a hoax, as claimed by climate change deniers.

It also casts doubt on the assertion of some scientists that there is no global warming hiatus.

Climatologists have lately realized a global warming hiatus, which is a period of relatively little change in globally averaged surface temperatures, now recurs at an average of every 15 years. These pauses have no effect on the long-term warming trend, however.

The reasons for these regular pauses, which before the advent of anthropogenic climate change averaged every 30 years, have been and are being investigated.

In 2013, the IPCC Fifth Assessment Report defined a global warming hiatus as a much smaller increasing linear trend over the 15 years from 1998 to 2012 compared to the 60 years from 1951 to 2012. Many studies over the years have looked into the possible causes of the puzzling short-term slowdown.

Studies disclosed in July 2015 based on updated datasets from the U.S. National Oceanographic Atmospheric Administration (NOAA) cast doubt on the existence of a global warming hiatus, and found no indication of a slowdown in earlier years. Some climate scientists, however, keep questioning this claim, arguing recent corrections in data do not negate the existence of a hiatus.

One of the latest studies into the global warming hiatus supports the latter contention. This study was conducted by Dr. Changyu Li, Prof. Jianping Huang and their colleagues, and a group of researchers from the Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University in China. The study affirmed that because of industrialization, the carbon dioxide in the Earth's atmosphere rose non-stop over the past 100 years. This rise is widely accepted as the main reason behind global warming.

On the other hand, the observational global mean atmospheric temperature surprisingly leveled off over the first decade of the 21st century. This slowdown, or the global warming hiatus, stands in marked contrast to the rapid warming during the late 20th century.

It's drawn a great attention worldwide owing to its ostensible contradiction of anthropogenic global warming.

The Chinese study contended the changes in ocean heat content might have a tight relationship with the atmospheric warming slowdown. It explores the energy redistribution between the atmosphere and ocean at different latitudes and depths by using observational data and simulations of a coupled atmosphere-ocean box model.

"Imagine the energy transport in our climate system as a water flow," Dr. Li said.

"Let's turn on a tap at the top of the system, the feed rate of which represents the top-of-the-atmosphere radiative imbalance caused by the greenhouse effect. A bucket below the tap can be an analogue of our atmosphere, and its water level is analogous to atmospheric warming. There is also a sinking flow at its bottom, draining into a larger bucket (i.e. the ocean). Now, here comes the key point. Generally, the water level of the atmospheric bucket rises as a result of global warming. However, if the drain rate approximately equals the feed rate of the tap, the water level of the bucket will not increase (the occurrence of the warming slowdown). That's the basic idea of our coupled box model."

Dr. Li explained a rapid increase in the global ocean heat content at a rate of some 9.8 × 1021 J yr-1 was detected in observations during the warming slowdown period. This means that from the energy point of view, there is no slowdown in global warming if we take the ocean into consideration.

He also added the increase of heat content provides a worrisome picture of the health of our oceans. This rapid oceanic warming will lead to serious degradation of marine ecosystems, and will eventually become "a great threat to the ocean biodiversity."

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