Understandable Earth Science

The IPCC 2014 synthesis report was approved by the IPCC on 1st November 2014.

The draft report (i.e. still needs copy-editing and formatting) is available here, if you want to read it. It is 116 pages long. The related press release is available here:

I thought it might be useful to summarise some of the more important points of the report for anybody who is interested but doesn’t have the time or inclination to wade through the entire document.

This first blog post explains the first diagram in the report. It is in the section “Topic 1: Observed changes and their causes” and it shows the real, measured changes that have been taking place on Earth due to global warming. The report states “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen.”

Figure 1.1 of the report presents 5 different sets of information – I have split the diagram up into individual parts and will explain each part separately. (Apologies for the poor quality of the diagrams – I used print screen to take them from the pdf of the report – hope the IPCC don’t mind me doing that – all images by IPCC.)

Panel (a):  Global temperature  change

Let’s start with panel (a). This shows the average temperature over time between the years 1850 and 2012. The temperature is plotted as the difference in temperature compared to the average (mean) of the years 1986–2005. So, looking at the diagram, between 1850 and 1900, the average temperature was ~ 0.6 °C colder than it was between 1986-2005, and nowadays it is about 0.2 degrees warmer than 1986-2005. The different coloured lines (black, blue and orange) correspond to different data sets and in the bottom panel, the grey boxes are an estimate of the uncertainty on the mean for one of the data sets. If you want to check out the data sources and how they are measured, the black line is data from the Met Office Hadley Centre and Climatic Research Unit, the blue line is from the NOAA (US National Oceanic and Atmospheric Administration) National Data Centre, and the orange line is from the NASA Goddard Institute. Importantly, all 3 data sets show good agreement. Annoyingly, the current report doesn’t actually list the data sets, and the links it provides in the figure caption are broken, but the same graph is shown on the Met-Office website, which lists the data sources.

The top panel shows the temperatures averaged over a single year, and you can see that yes, there is some variation – some years are colder than others. But then look at the bottom panel – this is the temperatures averaged over a decade. This is the more important diagram when you are looking at long term trends, because averaging over decades smoothes out any variability due to short-lived processes, like El Niño / El Niña events, cooling from volcanic eruptions, and temporarily reduced emissions from recessions. This bottom graph clearly shows that warming started in the early 1900s, paused during the mid 20th century, and then dramatically increased during the later part of the 20th century and into the 21st century.

Now, also consider that this data only shows up until 2012. Looking at data from the UK Metereological Office shows that average global temperature for last year, 2013, was 0.05 °C warmer than it was in 2012 and 2014 is already recording temperatures well above average (see here, here and here)

Summary: Average global temperatures are rising over time. Global warming is happening – absolutely no doubt about it!

Moving on to panel (b):

Global temperature change

This map shows which parts of the Earth are warming and which are cooling. You can see that the data set isn’t complete – we are missing much of the Arctic and Antarctic, large parts of the Pacific Ocean, and some central continental areas. These areas were excluded because the data record was less than 70 % complete and / or the first and last 10% of the time period had less than 20% data availability – i.e. the map is based on data that is robust and unlikely to be influenced by random outliers at the beginning and end of the time period. The little “+” symbols indicate grid squares where the data shows a statistically significant warming trend. Coloured squares without a “+” are not as statistically robust.  Even without a complete global map of data, we can see that most places are warming (yellow, orange, red, purple). Continental areas have warmed by as much as 2.5 °C over 111 years between 1901 and 2012. The only place that seems to have cooled  (pale blue) at all is a small patch of the north Atlantic (which I would hazard a guess is related to changes in thermohaline circulation of the Gulf Stream). The data on this map is surface temperature and it is derived from the orange data (NASA) in panel a.

Summary: The temperature rise over the last 111 years was not evenly distributed across the Earth. The vast majority of the Earth’s surface has seen a rise in temperatures.

Panel (c):

Change in Sea Ice

This shows the extent of summer Arctic sea ice since 1900 (averaged for each year over 3 months) and the extent of summer Antarctic sea ice since the late 1970s (averaged for each year over 1 month). The areal extent of summer Arctic sea ice has almost halved since the 1950s and is still reducing. At the moment the Antarctic summer sea ice looks like it hasn’t changed much, but we don’t know how extensive the ice was earlier in the 20th Century. Again, the different  coloured lines represent different data sets, and they show good agreement (at least in pattern, if not in absolute value for Antarctica). Unfortunately the links in the current draft of the IPCC report don’t work so I can’t tell you where the data comes from at the moment.

Summary: The areal extent of summer sea ice in the Arctic has almost halved since 1960.

Panel (d):

Global Sea Level Rise

This shows the change in global mean sea level between 1900 and 2010. Like panel (a), it is plotted as difference in sea level relative to the average sea level between 1986 and 2005. So, looking at the graph, back in 1900, sea level was about 0.15 m (15 cm) lower than the average for 1986–2005, and in 2010 it was about 0.05 (5 cm) higher than the average for 1986-2005. The 1986 – 2005 mean value is based on data from the longest running (i.e. most complete) data set.

Summary: Global mean sea level has risen around 20 cm since the year 1900.

Panel (e):

Change in precipitation

This map shows how on-land precipitation (rain and snow fall) has changed over time since 1951. The data used in this diagram were assessed and included using the same criteria as in panel (b) – i.e. data available for > 70% of the time period and with > 20% of the data available in the first and last 10% of the time period. The scale on this map is a bit confusing at first glance – it is precipitation in millimetres per year per decade. This isn’t an absolute change in value, like for the map in panel (b); this is plotting a change in the rate of change. In simple terms, the darkest blue shade represents areas where, every decade, the amount of precipitation has increased by 50 – 100 mm per year – i.e. the average precipitation per year between 1961-1971 was between 50 and 100 mm higher than the average precipitation per year between 1951-1961. And the average precipitation per year between 1971-1981 was between 100 and 200 mm higher than 1951-1961. Looking at the map, large parts of the African and Asian continents are drying out and seeing much less precipitation (incidentally, note the area of North Africa that hasn’t seen much change in precipitation levels – remember that this is the Sahara Desert – it can’t dry out much more…), while Northern Europe, much of the mainland USA, the east coast of South America and northern Australia are all seeing increased precipitation.

Summary: Patterns of rain and snowfall are changing, with some areas receiving much more precipitation and some receiving less.

So, that is the end of Figure 1.1 of the IPCC 2014 synthesis report.

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