What Is All the Fuss Over the Last Decade of Global Average Temperature?

May 22, 2013 | 4:57 pm
Brenda Ekwurzel
Senior Climate Scientist, Director of Climate Science

There’s been a flurry of magazine articles, a Congressional opinion piece in a national newspaper, and blogs disparaging climate models, all due to global average temperature not following a steady upward trend every step of the way. What’s remarkable is the sense these pieces convey that if there is a wiggle or pause over a decade in the clear long-term upward trend over the last century, then we should “throw out the baby with the bathwater.”

Factors that nudge global average temperature

Global average temperature rise is more like stairs with platforms than an escalator

Scientists would not expect to see a global average temperature trend that looked like an escalator. Rather they expect it to look more like staircases with periodic platforms. The point is the top of the stairs is higher than the bottom no matter how you get there and the same is true with global average temperature. Image source: Wikimedia Commons – Arpingstone

In reality, scientists know and study all the potential factors that influence global average temperature, and would not expect a straight-line trend. In fact, the wiggles and wobbles of the global annual average, which combines ocean and atmospheric temperatures, provide scientists fodder for understanding how the atmosphere, ocean, biosphere, etc. respond on different time scales to all the major and minor forces nudging them to and fro, including human-induced climate change.

People often picture climate change as a steady, unfaltering rise in temperature. But that’s misleading. Not every day or every month or every year is going to be hotter than the last; there’s going to be a lot of variation. But over time, the world has warmed and is absolutely going to continue getting hotter with continued heat-trapping gas emissions.

Global temperature trends, therefore, are going to look more like the staircase, rather than the escalator.

What’s up with global temperature since 2000?

So what are the factors at play over the last decade? Natural ocean cycles can cause a lot of the wiggles in the temperature trends and are one of the many sources of uncertainty in climate models. Tiny particles from incomplete combustion of fossil fuels can offset warming. Also, black particles can absorb sunlight and warm where they are suspended temporarily in the atmosphere. Volcanic eruptions over the last decade may have had an effect. The ocean absorbs most of the excess heat and serves as a buffer for atmospheric surface temperature, so upper ocean and deeper ocean warming trends are another factor considered by scientists in the last decade.

Then there is climate sensitivity – the hypothetical surface warming if carbon dioxide in the atmosphere is twice the pre-industrial level. Even as we grapple with reducing that uncertainty we know enough already that the Earth is sensitive to overloading the atmosphere with carbon from past climate alone. Orbital shifts initiated warming during past ice ages, which in turn unleashed carbon stored in different reservoirs. The bulk of the warming after ice ages occurred in response to extra carbon in the atmosphere released from thawing frozen landscape and warming oceans.

Climate Models and Past Climate Shed Light on Trends

Everyone from the Federal Reserve to the Oakland A’s uses modeling, whether it’s to set interest rates or find the best left-handed pitcher. No model is perfect, but they can all be useful and help us make better-informed decisions. Scientists will continue to improve climate models – and they keep getting better and better.  These models incorporate data on both natural factors (e.g. volcanic eruptions, the sun’s output) and human activities (e.g. overloading the atmosphere with carbon, tiny pollution particles).

When it comes to climate change, models are an important tool we have for anticipating future warming. We don’t have the luxury of running experiments on other planets to see what will happen to Earth, so instead we simulate our planet in models, using past climate data to make sure they’re matching reality. Even if we did not model the future, we have ample evidence archived in ice cores, sediment cores, rocks, and dead plants and animals to understand the factors that matter most for climate shifts at given time periods in Earth’s history. Sometimes the position of the Earth’s axis and orbit around the sun takes the alpha dog position, other times it is the movement of the plates and the positions of the continents that matter most, and at times like today our human activities have an outsized influence on global average temperature.

Regardless, much of the public criticism of climate models and global temperature is just more cherry-picking. It reminds me of the famous fable of the blind men that come across different parts of an elephant and create their own version of reality. The last decade is similar to only seeing the ear of the elephant. The big elephant in the room is the last several decades have gotten increasingly hotter and that we face a fundamental choice about how much hotter we will allow our planet to become as we overload the atmosphere with even more carbon.