New study confirms NOAA finding of faster global warming
Thomas Karl and colleagues were harassed by Republicans for publishing inconvenient science. A new study proves them right.
A new study has shown that a 2015 NOAA paper finding that the Earth is warming more rapidly than previously thought was correct. http://advances.sciencemag.org/content/3/1/e1601207
Once again, science is shown to work. The laborious process in which scientists check and recheck their work and subject their ideas to peer review has led to another success. An independent test of global warming data has confirmed a groundbreaking 2015 study that showed warming was faster than prior estimates.
Because of its inconvenient findings, the study’s lead author Thomas Karl was subjected to harassment by Lamar Smith (R-TX), chair of the House Science Committee, in an effort to impugn his credibility. But now Karl and his co-authors have been vindicated.
Let’s take a step back and discuss the science. Measuring the temperature of the Earth is hard. There are many locations to measure and many choices to make. Should we measure the temperature of the ground? Of the ocean waters? How deep in the water? If we measure air temperatures, what height should the measurements be taken? How many locations should we make measurements at? What happens if the instruments change over time or if the location changes? What happens if a city grows near a measurement location and the so-called urban heat-island effect grows? How do we estimate the temperatures in areas where no measurements exist?
These and many other questions make measuring global warming challenge. Different groups of scientists make different decisions so that depending on the institution, they will get a slightly different temperature result.
But this diversity is also a good thing. It turns out that it doesn’t matter whose results you use – NASA, NOAA, The Hadley Centre in the UK, the Japanese Meteorological Agency, or the Berkeley Earth group – they all report a warming world. However, the rates are slightly different. So, one persistent question is, which group is most accurate? Whose methods are best?
The new study looks into just this question. The group focused on perhaps the biggest differences among the groups – how they handle ocean temperatures. Specifically, global temperature values typically use a combination of near-surface air temperatures in land regions and sea surface temperatures in ocean regions. Since oceans cover approximately 70% of our planet, the way ocean temperatures are dealt with can separate the warming rates between these groups.
Ocean temperatures can be measured by ship-based temperature sensors, by special floating measuring instruments, or by satellites. Prior to the advent of satellites and floating sensors, ships were the main temperature sensing platforms. Ship sensors, which measure engine intake water, are known to be slightly warmer than the actual water. So using them introduces a warm bias in the measurements.
Also, as ships have gotten larger, the depth of the engine intakes have increased – meaning the tested water was further from the actual ocean surface. Since the temperature results from buoys differs from ship measurements, the various scientific groups have tended to try to perform corrections between the different sensors. The way the correction is done affects the reported warming rate.
The authors recognized that one of the biggest questions is how to stitch together different temperature results from different sensors. Therefore, they broke the temperature data up into groups according to the measurement device (buoys, satellites, ARGO floats, ships, etc.) and they evaluated warming rates separately for each group. The authors also used advanced statistics to handle areas where no data were recorded.
After applying their tests, the authors found that the results promoted by Karl at NOAA are the best, and other groups, in particular the Hadley Centre in the UK and the Japanese agency, are too cold.
So what does this all mean? A few things. First, this study is a nice reminder that the proper way for science to work is for publications to be scrutinized and checked by others. This process leads the entire scientific community to a deeper understanding of the science.
Second, this validates the scientists who were originally attacked by political non-scientists and in some cases by contrarian scientists. For instance, Judith Curry, a well-known critic of mainstream climate science was quoted as saying:
The new NOAA dataset disagrees with a UK dataset, which is generally regarded as the gold standard for global sea surface temperature datasets … The new dataset also disagrees with ARGO buoys and satellite analyses … Color me unconvinced.
I actually study ocean temperatures so I knew this statement by Judith Curry was complete nonsense. It is nice to see a team actually take the time to prove it. Perhaps she and others will finally admit they were wrong.
This paper is another reminder why it is so important to invest in the temperature measurements that are needed to create long-term climate records. We really need uninterrupted measurements that span many years/decades if we want to truly understand the Earth’s changing climate. Even though the costs of making these measurements are very small compared to what we spend on other less important activities, I am concerned that the new US administration will decide to pull the plug on these projects. If that happens, we will be flying blind.
Finally, and for those who read my posts regularly, I am sounding like a broken record. Global warming is happening, it never stopped, it never paused, and the models have gotten it right.
It reminds me of a debate between creationists and scientists. One scientist whose name I cannot remember stated, “we have the fossil record, we win.” Well, a similar quote works here. “We have the data, we win.” Now let’s move on to solving the problem.
Scientists say the global ocean circulation may be more vulnerable to shutdown than we thought
Intense future climate change could have a far different impact on the world than current models predict, suggests a thought-provoking new study just out in the journal Science Advances. If atmospheric carbon dioxide concentrations were to double in the future, it finds, a major ocean current — one that helps regulate climate and weather patterns all over the world — could collapse. And that could paint a very different picture of the future than what we’ve assumed so far.
The Atlantic meridional overturning circulation, or AMOC, is often described as a large oceanic conveyor belt. It’s a system of water currents that transports warm water northward from the Atlantic toward the Arctic, contributing to the mild climate conditions found in places like Western Europe. In the Northern Atlantic, the northward flowing surface water eventually cools and sinks down toward the bottom of the ocean, and another current brings that cooler water back down south again. The whole process is part of a much larger system of overturning currents that circulates all over the world, from pole to pole.
But some scientists have begun to worry that the AMOC isn’t accurately represented in current climate models. They say that many models portray the current as being more stable than real-life observations suggest it actually is. Recent studies have suggested that the AMOC is weakening, although there’s some scientific debate about how much of this has been caused by human activities and how much by natural variations.
Nevertheless, the authors of the new study point out, many climate models assume a fairly stable AMOC — and that could be affecting the predictions they make for how the ocean will change under future climate change. And because overturning circulation patterns have such a significant effect on climate and weather all over the world, this could have big implications for all kinds of other climate-related projections as well.
“This is a very common and well-known issue in climate models,” said the new study’s lead author, Wei Liu, a postdoctoral associate at Yale University, who conducted the work while at the University of California at San Diego. “I wanted to see, if I use a corrected model, how this will affect the future climate change.”
Liu and colleagues from the UC-San Diego and the University of Wisconsin at Madison took a commonly used climate model and corrected for what they considered to be the AMOC stability bias. Then they ran an experiment to see how the correction would affect the model’s projections under future climate change. They instantaneously doubled the atmospheric carbon dioxide concentration from present-day levels in both the corrected and uncorrected models, and then they let both models run for hundreds of simulated years.
The differences were striking. In the uncorrected climate model, the AMOC weakens for a while, but eventually recovers. In the corrected model, however, the AMOC continues to weaken and after 300 years, it collapses altogether.
In a commentary also published today in RealClimate, Stefan Rahmstorf, an oceans physics expert at the Potsdam Institute for Climate Impact Research, explained how such a collapse could occur when the AMOC gets too weak.
“Freshwater continually flows into the northern Atlantic through precipitation, rivers and ice-melting,” he wrote. “But supply of salty waters from the south, through the Gulf Stream System, balances this. If however the current slows, there is less salt supply, and the surface ocean gets less salty.”
Because freshwater is less dense than salty water, this process can lead to a kind of stratification, in which the lighter freshwater gets stuck on the surface of the ocean and can’t sink to the bottom when it reaches the cooler north. When this happens, the overturning process that drives the current back down south again can’t occur.
“There is a critical point when this becomes an unstoppable vicious circle,” Rahmstorf wrote. “This is one of the classic tipping points in the climate system.”
The resulting climate consequences, compared to the uncorrected model, are also dramatic. Without the usual transport of warm water into the north, the corrected model predicts a marked cooling over the northern Atlantic, including in the United Kingdom, Iceland and northwestern Europe, as well as in the Arctic, where sea ice begins to expand.
Because the AMOC is part of a larger global conveyor system, which ferries warm and cold currents between the equator and both poles, the model predicts disruptions in other parts of the world as well. Without cold water moving back down south again, the corrected model indicates a stronger warming pattern south of the equator than what’s predicted by the uncorrected model, causing a polarization in precipitation patterns over the Americas — more rain for places like northeastern Brazil and less rain for Central America. The model also predicts a greater reduction in sea ice for the Antarctic.
All this doesn’t necessarily mean that everything we thought we knew about the future climate is wrong. For one thing, most modern climate projections focus on the next few decades or so, noted Thomas Haine, an expert on ocean circulation at Johns Hopkins University. And within 50 years or so, both the uncorrected and corrected models in the new study produce similar results. It is only after that, under extreme warming, that the current shifts.
Liu also cautioned that certain aspects of the experiment can’t exactly be considered realistic — for instance, instantaneously doubling the atmospheric carbon dioxide concentration. Current climate efforts are aimed at keeping us from ever getting to such a point — but even if we did, the process would happen gradually, not overnight. So the model’s outcome might have been different if the researchers had adopted a more realistic scenario.
Haine also suggested that the correction in the new study may have actually been a bit too strong, compared to actual observations — in other words, the modeled AMOC is “probably more unstable than the real system,” he said.
Rahmstorf also pointed out this issue in his commentary — but he added that the climate model used also did not account for an influx of meltwater from Greenland under future climate change, an event that recent research suggests could substantially speed the AMOC’s weakening.
“With unmitigated emissions . . . the Gulf Stream System weakens on average by 37 percent by the year 2300 without Greenland melt,” he notes. “With Greenland meltwater this doubles to 74 percent. And a few months ago, a study with a high-resolution ocean model appeared, suggesting that the meltwater from Greenland is likely to weaken the AMOC considerably within a few decades.”
The fact that current models don’t take this melting into account is further support for the idea that scientists have been underestimating the risk of a future AMOC collapse, he suggested.
According to Liu, the new study serves to make a point about the dramatic effects that can occur when corrections are made in climate models, as well as the AMOC’s major role in the global climate. By tweaking a climate model to make it more consistent with real-life observations, very different outcomes may be observed, Liu noted.
“I would say that it is reasonably well-accepted that a current generation of climate models [is] missing the essential physics in representing the AMOC,” said Haine. And he added that the new study “points to the need to fix these biases in the climate models.