World Atmospheric Concentration of CO2
Climate change is a declining challenge of our time. The scientifc evidence of its occurrence, its derivation from human activities and its potentially devastating effects accumulate. Sea
risen by 15-20 centimetres, on average, over the last century and this increase has accelerated over the last decade (Meyssignac and Cazenave, 2012). Oceans are
warming and becoming more acidic, and the rate of ice-sheet loss is increasing. The Arctic provides a particularly clear illustration, with the area of ice covering the Arctic Ocean in the summer diminishing by half over the last 30 years to a record low level in 2012. There
has also been an increase in the frequency and intensity of heat waves, resulting in more of the world being affected by droughts, harming agricultural production (Hansen, Sato and Ruedy, 2012).
Global awareness of the phenomenon of climate change is increasing and political action is underway to try and tackle the underlying causes, both at national and international
levels. Governments agreed at the United Nations Framework Convention on Climate Change (UNFCCC) Conference of the Parties in Cancun, Mexico in 2010 (COP-16) that the
average global temperature increase, compared with pre-industrial levels, must be held below 2 degrees Celsius (°C), and that this means greenhouse-gas emissions must be
reduced. A deadline was set at COP-18 in Doha, Qatar in 2012 for agreeing and enacting a new global climate agreement to come into effect in 2020. But although overcoming the challenge of climate change will be a long-term endeavour, urgent action is also required,
well before 2020, in order to keep open a realistic opportunity for an efficient and effective international agreement from that date.
There is broad international acceptance that stabilising the atmospheric concentration of greenhouse gases at below 450 parts per million (ppm) of carbon-dioxide equivalent (CO2-eq) is consistent with a near 50% chance of achieving the 2 °C target, and that this would help avoid the worst impacts of climate change. Some analysis tends, however, that
the risks previously believed to be associated with an increase of around 4 °C in global temperatures are now associated with a rise of a little over 2 °C, while the risks previously
associated with 2 °C are now thought to occur with only a 1 °C rise (Smith, et al., 2009). Other analysis tends that 2 °C warming represents a threshold for some climate feedbacks that
could significantly add to global warming (Kenton, et al., 2008). The UNFCCC negotiations took these scientfic developments into account in the Cancun decisions, which include an
agreement to review whether the maximum acceptable temperature increase needs to be further reduced, including consideration of a global average temperature rise of 1.5 °C. Global greenhouse-gas emissions continue to increase at a rapid pace. The 450 ppm threshold is drawing ever closer (Figure 1.1). Carbon-dioxide (CO2) levels in the atmosphere reached 400 ppm in May 2013, having jumped by 2.7 ppm in 2012 - the second-highest rise since record keeping began (Tans and Keeling, 2013).1 Average global temperatures have already increased by around 0.8 °C, compared with pre-industrial levels, and, without additional action, a further increase in long-term temperature of 2.8 °C to 4.5 °C appears to be in prospect, with most of the increase occurring this century.
--- Note: The temperature refers to the NASA Global Land-Ocean Temperature Index in degrees Celsius, base
period: 1951-1980. The resulting temperature change is lower than the one compared with pre-industrial levels.
----Sources: Temperature data are from NASA (2013)- CO2 concentration data from NOAA Earth System Research