Long description

This figure consists of two panels. The first panel shows the atmospheric concentration of methane and solar radiation from the Milankovitch cycle. The horizontal axis is time measured in years before present (BP) and the minimum is 250 000 years BP with ticks every 50 000 years. The maximum on this axis is 0 which is the present day. The left-hand vertical axis is methane in the EPICA ice core in parts per billion (ppb): the minimum is 300 ppb and the maximum is 900 ppb with ticks every 100 ppb. The right-hand vertical axis is solar radiation in watts per square metre: the minimum is 440 watts per square metre and the maximum is 540 watts per square metre with ticks every 20 watts per square metre. The panel has a blue-dashed line which shows the solar radiation and it is a smoothly varying curve cycling from about 440 watts per square metre to 510 watts per square metre on a period of approximately 25 000 years. An orange line shows the methane concentration in the EPICA ice core. Although it is not smoothly varying, methane concentration does cycle in a similar way to the solar radiation between the values of about 400 ppb and 600-700 ppb, such that high solar radiation corresponds to high methane, and low solar radiation corresponds to low methane. This is so throughout virtually the entire record, although this pattern is broken in the last 5 000 years, where methane levels do not reduce and instead rapidly increase to around 900 ppb. The second panel shows just methane concentration over the last 11 500 years. The horizontal axis is time measured in years before present (BP). The minimum is 11 500 years BP with ticks at 10 000 and 5 000 years BP. The maximum on this axis is 0 which is the present day. The vertical axis is methane concentration in parts per billion (ppb): the minimum is around 410 ppb and the maximum is 800 ppb with ticks every 50 ppb from 450 to 750 ppb. From 11 500 to 5 000 years BP the methane concentration in the core is on a downwards trend from approximately 700 ppb to 550 ppb. The trend is what would be expected from the previous panel and after 5000 years BP the trend is marked with an arrow continuing downwards reaching approximately 450 ppb at 0 on the horizontal axis. However, the methane concentration departs from this trend at 5 000 years BP and reverses direction to become a virtual mirror image of the previous 5 000 years, reaching greater than 700 ppb at 0 on the horizontal axis. So there is a departure from the expected trend of methane of the last 250 000 years.