For those of us who are concerned about global warming, two of the most critical questions we ask are, “how fast is the Earth warming?” and “how much will it warm in the future?”.
The first question can be answered in a number of ways. For instance, we can actually measure the rate of energy increase in the Earth’s system (primarily through measuring changing ocean temperatures). Alternatively, we can measure changes in the net inflow of heat at the top of the atmosphere using satellites. We can also measure the rate of sea-level rise to get an estimate of the warming rate.
Since much of sea-level rise is caused by thermal expansion of water, knowledge of the water-level rise allows us to deduce the warming rate. We can also use climate models (which are sophisticated computer calculations of the Earth’s climate) or our knowledge from Earth’s past (paleoclimatology).
Many studies use combinations of these study methods to attain estimates and typically the estimates are that the planet is warming at a rate of perhaps 0.5 to 1 Watt per square meter of Earth’s surface area. However, there is some discrepancy among the actual numbers.
So assuming we know how much heat is being accumulated by the Earth, how can we predict what the future climate will be? The main tool for this is climate models (although there are other independent ways we can study the future). With climate models, we can play “what-if scenarios” and input either current conditions or hypothetical conditions and watch the Earth’s climate evolve within the simulation.
Two incorrect but nevertheless consistent denial arguments are that the Earth isn’t warming and that climate models are inaccurate. A new study, published by Kevin Trenberth, Lijing Cheng, and others (I was also an author) answers these questions.
The study was just published in the journal Ocean Sciences; a draft of it is available here. In this study, we did a few new things. First, we presented a new estimate of ocean heating throughout its full depth (most studies only consider the top portion of the ocean). Second, we used a new technique to learn about ocean temperature changes in areas where there are very few measurements. Finally, we used a large group of computer models to predict warming rates, and we found excellent agreement between the predictions and the measurements.
According to the measurements, the Earth has gained 0.46 Watts per square meter between 1970 and 2005. Since, 1992 the rate is higher (0.75 Watts per square meter) and therefore shows an acceleration of the warming. To put this in perspective, this is the equivalent of 5,400,000,000,000 (or 5,400 billion) 60-watt light bulbs running continuously day and night. In my view, these numbers are the most accurate measurements of the rate at which the Earth is warming.
What about the next question – how did the models do? Amazingly well. From 1970 through 2005, the models on average showed a warming of 0.41 Watts per square meter and from 1992-2005 the models gave 0.77 Watts per meter squared. This means that since 1992, the models have been within 3 % of the measurements. In my mind, this agreement is the strongest vindication of the models ever found, and in fact, in our study we suggest that matches between climate models and ocean warming should be a major test of the models.
Despite these excellent results, scientists want to do better. During a conversation with Dr. Trenberth, he told me:
Progress is being made on understanding the energy flows through the climate system as datasets are improved and methods of analyzing the data are being revised and rigorously tested. We can never go back and make observations that were missed, but we can still improve knowledge of how the climate has evolved, even in recent (post-2005) data-rich (Argo) times.
My other colleague, Dr. Lijing Cheng says:
Ocean heat content is a vital climate indicator and is a key metric for global warming. How well ocean heating can be assessed by observations and can be simulated by climate models are a cornerstone of climate studies. By collecting the state-of-the-art observational ocean warming estimates and climate model results, this study gives the current status of our warming world and its future heating. We will continue to work hard to improve both measurements and models to better understand the climate change.
Readers should also know that our study isn’t the only one of its kind to make these findings. A paper published before ours by a world-class group of scientists came to similar conclusions. So too does another study found here. When multiple and independent studies come to similar conclusions, it suggests that the conclusions are robust.
Our current warming and future predictions are so very important to understanding this very important topic. Fortunately, this new study advances our knowledge in these areas.
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