Via NoTricksZone we learn of a new study showing that over the past 20 years the largest influence on climate has been changes in cloud cover. In fact two such studies have come out recently showing the same thing, so we will discuss them over two weeks. And they both raise the same point. Changes in greenhouse gas levels have been too small over the past 20 years to explain the run up of temperatures, especially around 2016. Most of it can be attributed to changes in the amount and characteristics of clouds. So why did cloud cover change? That’s the real question.

At a simple level, the Earth maintains a balance of incoming energy from the sun and outgoing energy in the form of heat radiation. The incoming energy is shortwave (SW) and the outgoing energy is longwave (LW). It’s not just straight in and straight out, though, the energy flows bounce around inside the climate system based on cloud cover, reflectivity of the surface, the amount of greenhouse gases and so forth. Satellites measure the energy flows at the top of the atmosphere and the net absorption at the surface. The data can be used to detect if there is an imbalance between the inflows and the outflows, which they call the Earth Energy Imbalance. If it’s a positive imbalance, the climate system will warm up, and if it’s negative, we’ll get cooling.

The new Loeb et al. study concluded that there has been an increase in the net incoming radiation being absorbed at the surface. The outgoing LW radiation has increased but so has the incoming shortwave radiation, by a larger amount. Meanwhile the sun isn’t any brighter. So how can there have been more incoming SW radiation?

Part of the answer is that the surface got a bit less reflective, but the biggest factor (62%) was a change in cloud cover. Added to that was an increase in the amount of water vapour in the atmosphere, which acts like a very powerful greenhouse gas.

So what caused the change in clouds and water vapour? The authors, thankfully, don’t simply shrug and blame it on you wanting to heat your home or drive your car to work. Instead they note that the biggest changes happened starting around 2014, which is when the Pacific Decadal Oscillation (PDO) flipped into a positive mode. The PDO is one of those large-scale cyclical patterns governing the oceans and atmosphere that no one seems to understand and no one can predict, but it drives a lot of the changes we experience in the weather. When the PDO flipped into a positive mode in 2014, the result was a burst of warming a short time later. And, the authors note, “We can therefore expect even greater changes in climate in the coming decades if internal variability associated with the PDO remains the same. If the PDO were to reverse in the future, that reversal would likely act to decrease the rate of heat uptake.”

And if you’re curious what the PDO has been doing lately, the authors helpfully provide a diagram.

It might be a good time to stock up on heating fuel. If you can find any.