Good, did that get your attention? I'm interrupting my "Six Degrees" series of posts to address a topic that has been in the news recently. That topic is the supposedly new observation or conclusion that the planet is cooling, in opposition to the warming trend. For example, the BBC ran this piece entitled "What happened to global warming?". The answer, in a nutshell, is NOTHING. The article has stirred up quite a bit of argument and discussion between proponents and skeptics of global warming. So what's all the fuss about? I'll try to explain. But first, let me be blunt: the natural world does not give a fig about our opinions, whether we support ("believe in") or deny (see here) human-caused global warming.
Okay, so what is this about? The Earth's climate is a dynamic system, meaning that it evolves over time, and at any given time may be considered to be in a particular state. One of the major drivers or controllers of climate is the world ocean, primarily because the ocean is the planet's major heat sink. This is the result of water's high heat capacity. To understand this, think about a pot of water boiling on your stove. You watch it, and it just sits there. Now suppose one stuck one's hand into the flame (Readers: Do not try this at home). Gee, your hand heated up pretty quickly! Not so with the water. But eventually, the water absorbs enough heat energy to begin it's transition from the liquid state to a vapour state; that is, it boils. It takes a lot of energy to change the temperature of water. That's just one wonderful property due to water's fantastic molecular structure. The world ocean covers 70% of the planet's surface, and it's movement is the global movement of heat.
The oceans are dynamic; they flow, they mix, they churn. One could say that our understanding of long-term ocean dynamics is still very incomplete, but we do know that there is long-term variability in the states of the world ocean, and hence in climate. Perhaps the most familiar manifestation of this is the El Niño-La Niña quasi-cycle. Another is the less familiar Pacific Decadal Oscillation, or PDO. These phenomena represent natural variability in the ocean's circulation. The variation translates into climate variability. We've seen these cycles operate through the 20th century. The century began a bit on the warm side, but cooled globally, on average, from the 1940's through 1970's. The variation is caused by the ocean sloshing back and forth. Now the sloshing is causing cooling once more.
THIS IS UNRELATED TO THE TREND OF HUMAN-DRIVEN GLOBAL WARMING. Let me say this again: This is not a reversal, nor is it refutation of anthropogenic global warming. How do I know this? Science folks. The media really should speak with the scientists doing the research, rather than sensationalizing and immediately seeking the nonsensical sound bytes of dwellers on the fringe. The cooling has been noted for some time (I blogged about it almost a year ago!), and we know that it is being driven by the PDO. Now here's the cool part. We can tease apart this natural variability and other influences, so-called external forcings. This means that we can look at global temperature, and ask how much of it is due to the ocean driver, and how much to external factors. Should I tell you what the answer is? Can you guess? IT'S STILL WARMING. Overlain on the natural variability of our climate is a clear upward trend through the 20th century.
Don't believe me? Show your friendly neighbourhood scientists a little love and read one of the original studies here. 'Nuf said.