Courtesy of Amanda Leigh Mascarelli, originally published in Nature.

1. How much warming and by when

Although there is wide agreement that we will see a warming trend in atmospheric and sea surface temperatures over the next century, just how climate will change in the short term is less certain. One of the first studies to address this concluded that warming may slow for a decade before rapid climate change takes off (Nature 453, 84-88; 2008).

Noel Keenlyside of the Leibniz Institute of Marine Sciences, Germany, and colleagues found that owing to changes in ocean circulation that occur on decadal time scales, global average surface temperatures in parts of the ocean may not increase over the period from 2005 to 2015, compared with 2000 to 2010, and that some surface waters could even cool slightly.

Their findings do not imply that global warming is not happening, but instead that natural oscillations in the climate system could lead to short-term changes that temporarily eclipse manmade warming. However, unconvinced by the temperature forecasts of Keenlyside et al., a group of esteemed climate scientists on the Real Climate blog staked $5,000 against the prediction that falling temperatures in some regions will cause a slight slow down in global warming. We'll have to wait until 2015 to know for sure.

2. Where to stabilize

A muddy point that perhaps only became muddier in 2008 is the concentration at which we must stabilize atmospheric greenhouse gases to avert a dangerous degree of change. Atmospheric CO₂ concentrations today hover around 385 parts per million (p.p.m.), and many scientists have settled on 400 to 450 p.p.m. as the upper limit to keep warming below 2°C above pre-industrial levels. But NASA climatologist James Hansen is one of a group of scientists now saying that more stringent limits of approximately 350 p.p.m. will be necessary to avoid "irreversible catastrophic effects" (Hansen, J. et al. Columbia University 2008; http://www.columbia.edu/jeh1/2008/TargetCO2_20080407.pdf).

Others are veering in the opposite direction: in a report for the Australian government this year, economist Ross Garnaut reiterated Professor Nick Stern's 2006 recommendation to stabilize the atmospheric concentration of CO₂ at up to 550 p.p.m. (Garnaut, R. The Garnaut Climate Change Review; Cambridge University, 2008; http://www.garnautreview.org.au/index.htm). There may be remaining scientific uncertainty about just how much CO₂ is too much, to avoid disaster, but if the current state of play is anything to go by, reaching a final figure in any agreement will also be a question of what is politically achievable.

3. Where the missing carbon is going

Surprising as it sounds, scientists still do not have a clear grasp of where carbon is coming from, where it's going and in what amounts. Yet, under the Kyoto Protocol, developed nations that have ratified the agreement are to receive credits for sequestering carbon through improved land management practices and reforestation. About half of the CO₂ that wafts into the atmosphere from fossil fuel combustion is absorbed by the oceans, plants, forests and croplands, but how much of the carbon is swallowed up by the oceans versus land is still unclear. (Global Change Biol. 14, 2910-2922; 2008, Nature Geoscience 1, 569-570; 2008 and Eos Trans. AGU 89, doi:10.1029/2008EO430001; 2008).

One reason for the dearth of information is that ground-based monitoring stations are few and far between, and until now, the technology hasn't been available to obtain fine-scaled, precise measurements of CO₂ in the atmosphere. But the launch next year of two carbon-detecting satellites, NASA's Orbiting Carbon Observatory and the Japanese Greenhouse Gases Observing Satellite, should soon help to fill in this knowledge gap, which is critical to establishing a reliable carbon accounting system.

4. Whether warming worsens storms

The jury is still out on whether hurricanes will increase in intensity, frequency or duration as a result of global warming. Globally, the number of major hurricanes has shot up by 75 per cent since 1970, but the role of human activity in this rise has remained contentious. This year, new evidence has caused leading experts to reassess their positions on this key issue. Weighing in on the debate early on, Kerry Emanuel of the Massachusetts Institute of Technology used a bespoke model to show that warming should reduce the frequency of hurricanes globally, although hurricane intensity may increase in some locations (Bull. Am. Meteorol. Soc. 89, 347-367; 2008). Supporting these predictions was a paper in May (Nature Geosci. 1, 359-364; 2008) that projected fewer Atlantic hurricanes during the twenty-first century.

In September, James Elsner of Florida State University and colleagues concluded that in the Atlantic, the strongest tropical cyclones will grow even stronger in a warming world (Nature 455, 92-95; 2008). One explanation for the smattering of results is that scientists do not yet have a clear understanding of the relationship between sea surface temperature and hurricane formation on local or global scales. A team of researchers led by Gabriel Vecchi of the US National Oceanic and Atmospheric Administration recently summarized the difficulties in predicting hurricane activity (Science 322, 687-689; 2008). Ultimately, they argue that relative, rather than absolute, warming of regions such as the Atlantic Ocean is probably behind the recent surge in hurricane activity. And predictions show that relative warming will remain fairly constant throughout the remainder of this century. If their hypothesis is right, the worst hurricane seasons may already be behind us.

5. How fast Greenland is melting

One of the greatest wild cards in predicting how the climate system will respond to warming is the Greenland ice sheet. Complete melting of Greenland could raise sea level by seven metres and spell catastrophe for coastal cities and millions of inhabitants. Scientists previously assumed that this melting would happen gradually over 1,000 years or more, but this is being re-examined in light of new evidence, including a study showing that Greenland could experience rapid melting over centuries, rather than millennia, and that sea level could rise by 1.3 metres by 2100 as a result (Nature Geoscience 1, 620-624; 2008).

A recent study reported in Science looked closely at the possibility of large sea level rise by 2100 and concluded that sea level could rise by a maximum of two metres by 2100 if all variables were accelerated and pushed to the extreme, but that sea-level rise of 0.8 metres by 2100 is a more likely outcome (Science 321, 1340-1343; 2008). Scientists know that the Greenland Ice Sheet is undergoing dramatic melting, but accurately capturing its status in climate models and predicting future behaviour in response to warming is difficult due to a lack of long-term observations and data. In addition, scientists don't yet have a handle on ice sheet dynamics, including how subsurface melting contributes to slipping and sliding of the ice sheet.

 Two studies this year shed light on how meltwater may lubricate the bottom of the ice sheet, contributing to its slippage towards the ocean (Science 320, 778-781; 2008 and Science 320, 781-783; 2008). But by far the most disturbing question that remains is whether Greenland has already endured enough warming to push it to the point of no return.

Amanda Leigh Mascarelli is a freelance science writer based in Denver, Colorado.

Related Reading:
Our Climate: What We've Learned in 2008
Climate Change Ahead of Schedule

Image Credit:
Tavallai