What's the evidence that the world's warming up?
You may have heard stories from elderly relatives of ice-skating on rivers or trudging through snow in places where now even the coldest weather produces only a frost. But is this just exaggerated memory or has the climate really changed?
As anyone who has gone to a rained-out picnic knows, it's hard to judge the weather because it's a highly variable chaotic phenomenon. On the other hand, you can be confident that the winter months will be colder than those in the summer. The difference here is that between weather and climate, and the challenge has been to identify changes in the climate (if any) that go beyond the weather's natural variability.
Now, thanks to mounting observational evidence from large long-term datasets, it’s clear that the Earth has warmed significantly over the past century, and particularly so in recent decades.
Basic meteorological and tide records have been kept since the 19th century, and the 1957 International Geophysical Year spurred the establishment of programmes to continuously monitor the Earth system. Many of these programmes have since expanded, creating Earth observational networks like the Global Atmospheric Watch laboratories. Another example is the 3000 untethered ARGOS floats travelling with ocean currents recording seawater parameters.
But it was the introduction of satellite-based observations in the 1970s that really gave scientists what they needed - a detailed whole-Earth picture. And by overlaying surface and satellite observations a compelling picture emerges of a planet undergoing relatively fast climate change.
So, what are the physical changes we are seeing? The temperature of the air is a good place to start. Global average surface air temperatures have increased by about 0.8 degrees Celsius since the late 19th century. To put that in context, the temperature difference between ice ages and the warmest periods in Earth’s geological history is only about 10 degrees Celsius. Moreover, the rate of warming has been twice as fast in the last 50 years as over the last century. The atmosphere is getting hotter, and it’s getting hotter faster.
Warmer air holds more water vapour, which is changing global precipitation patterns. Some regions have more rain, such as eastern North and South America and northern Europe, while other areas are receiving less, including the Mediterranean, southern Africa, and some parts of southern Asia. Over land there has been a rise in extremely high rainfall events, and drought intensity has increased since the 1970s. Changing salinity suggests rainfall and evaporation has also altered over the oceans.
The surface oceans are becoming warmer too. And because warmer water is less dense, the oceans are expanding, causing about 1.6 mm/year of global sea-level rise over the last decade. Presently sea level is rising at the rate of about 3 mm/year. The additional sea-level change is due to the melting of the cryosphere, that is, glaciers, ice caps and so on.
On average, glaciers are retreating and snow cover is decreasing globally. Both the Antarctic and Greenland ice sheets are losing mass because they are melting more quickly than the snow accumulates, and the Arctic summer sea ice extent has shrunk by up to 9% per decade since the 1970s. Permafrost (soil that stays frozen year long) in the Northern Hemisphere is melting too – there has been a 7% loss in permafrost area over the last century.
But what makes the current changing temperature different from Earth’s history of ice age cycling? A century ago Arrhenius showed that CO2 in the atmosphere is a control of Earth’s temperature. All carbon on Earth is divided up between the biosphere, oceans, atmosphere, and lithosphere (eg. rocks).
Natural processes move carbon between these ‘reservoirs’ on timescales ranging from hundreds of years (e.g. forest growth) to millenia (e.g. rock weathering, volcanism). These exchanges produce rises and falls in levels of atmospheric CO2, and subsequently Earth’s temperature.
But by burning fossil fuels we move carbon from the lithosphere to the atmosphere. And we are doing it at a rate that completely overwhelms any natural rates of carbon exchange. The Earth’s current warming is, at least to a large extent, caused by us.
So, our observations tell us the planet is warming, changing the climate, and subsequently our weather. What will the Earth be like in the year 2050? In 2100? How will we adapt? There is some comfort in knowing that some of the smartest minds in the world are working on these very questions. But, the biggest challenge in terms of dealing with climate change is now not limited scientific understanding or technology, but rather the lack of political will.
- IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.