For those of us who worry about global warming, two of the most critical questions we ask are, “how fast is the Earth warming?” and “how warm will it get in the future?”.
The first question can be answered in several ways. For example, we can actually measure the rate of energy increase in the Earth system (primarily by measuring ocean temperature changes). Alternatively, we can measure changes in the net heat influx to the top of the atmosphere using satellites. We can also measure the rate of sea level rise to get an estimate of the rate of warming.
Since much of the rise in sea level is caused by the thermal expansion of water, knowing the rise in water level allows us to infer the rate of warming. We can also use climate models (which are sophisticated computer calculations of the Earth’s climate) or our knowledge of the Earth’s past (paleoclimatology).
Many studies use combinations of these study methods to obtain estimates and generally 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. However, there is some difference between the actual numbers.
So, assuming we know how much heat is stored by the Earth, how can we predict what the future climate will be like? The main tool for this is climate models (although there are other independent means of studying the future). With climate models, we can play “what-if scenarios” and enter either current conditions or hypothetical conditions and observe the evolution of the Earth’s climate in the simulation.
Two incorrect but nonetheless consistent denial arguments are that the Earth is not warming and that climate models are inaccurate. A new study published by Kevin TrenbergLijing Cheng, and others (I was also an author) answer these questions.
The study has just been published in the journal Ocean Sciences; a draft is available here. In this study, we have made some novelties. First, we presented a new estimate of the warming of the ocean over its entire depth (most studies only consider the upper part of the ocean). Second, we used a new technique to learn more about ocean temperature changes in areas where there are very few measurements. Finally, we used a large number of computer models to predict warming rates, and found excellent agreement between predictions and measurements.
According to measurements, the Earth gained 0.46 Watt per square meter between 1970 and 2005. Since 1992, the rate is higher (0.75 Watt per square meter) and therefore shows an acceleration of warming. To put that into perspective, that equates to 5,400,000,000,000 (or 5,400 billion) 60-watt light bulbs running continuously day and night. In my opinion, these numbers are the most accurate measures of how fast the Earth is warming.
What about the next question – how did the models perform? Surprisingly well. From 1970 to 2005 the models showed an average warming of 0.41 Watts per square meter and from 1992 to 2005 the models gave 0.77 Watts per square meter. This means that since 1992, the models are within 3% of the measurements. In my mind, this agreement is the strongest justification of the models ever found, and in fact, in our study, we suggest that correspondences between climate models and ocean warming should be a major test of the models.
Despite these excellent results, scientists want to do better. In a conversation with Dr. Trenbergh, he told me:
Progress is being made in understanding energy flows through the climate system as data sets are improved and data analysis methods are revised and rigorously tested. We can never go back and make observations that have been missed, but we can still improve knowledge of climate change, even in recent (post-2005) data-rich times (Argo)..
My other colleague, Dr Lijing Cheng says:
The heat content of the oceans is an essential climate indicator and a key measure of global warming. The extent to which ocean warming can be assessed by observations and simulated by climate models is the cornerstone of climate studies. By bringing together state-of-the-art observational estimates of ocean warming and results from climate models, this study gives the current state of our world’s warming and its future warming. We will continue to work hard to improve both measurements and models to better understand climate change.
Readers should also be aware that our study is not the only one of its kind to make these findings. A paper published before ours by a group of world-class scientists came to similar conclusions. The same is true for another study here. When multiple independent studies come to similar conclusions, it suggests that the conclusions are strong.
Our current warming and our future predictions are very important in understanding this very important subject. Fortunately, this new study advances our knowledge in these areas.