Understandable Earth Science

Posts tagged ‘IPCC’

Why we need to stop emitting CO₂ – A.S.A.P.!

In my first climate-related blog I talked about why I was so angry that the world wasn’t taking action to prevent climate change – 10 years ago the wedges concept, introduced  by Pacala and Socolow [1], gave a clear prediction of how our annual CO₂ emissions were increasing and identified ways to reduce emissions. Back then, the world was still thinking in terms of how CO₂ *emission rates* affect climate change. Since then we have realised that emission rates are relatively unimportant and that global cumulative CO₂ emissions are what we need to keep an eye on.

This is because CO₂ has a long residence time in the atmosphere – if we were to completely stop burning fossil fuels and emitting CO₂ today, it would take up to 35 thousand years for all of the CO₂ we emitted since the industrial revolution to be re-absorbed [2] (if you want to know more about that, there is a nice blog here). This means that our CO₂ emissions over decades, even centuries, are pretty much instantaneous on the carbon-cycle timescale.

CO₂ residence

Figure 1: Carbon is stored as fossil fuels for millions of years. When we take it out of the ground and burn it, we are releasing CO₂ instantaneously on geological timescales. It then takes tens of thousands of years for that CO₂ to be removed from the atmosphere and be stored long-term in the Earth.

The amount of global warming will be controlled by the amount of CO₂ hanging around in the atmosphere, and, because it hangs around in the atmosphere for so long, that is controlled by the total amount of CO₂ we have emitted, rather than how fast we are emitting it. In 2009, Allen et al [3] calculated the Earth will warm by ~2 °C for every 3.67 trillion tonnes of CO2 that we emit [4]. If we want a good chance of avoiding global temperature rise of more than 2 °C, the most CO2 we can emit is 3.67 trillion tonnes – total! period! ever!.

Reducing emission rates just delays the 2 °C temperature rise – it doesn’t prevent it. Stabilising emission rates buys us time but doesn’t solve the problem. The only way to solve the problem is to completely stop emitting CO₂.

This next diagram, simplified  from the IPCC 2014 report (Figure SPM.5, p9 [5]), is a nice illustration of  how much we can expect the global temperature to rise because of the total amount of CO₂ we emit. The x-axis shows cumulative CO₂ emissions since 1870 in gigatonnes of CO₂. (GtCO₂) The y-axis shows the temperature increase compared to pre-industrial temperatures. There are different ways of modelling the climate response to carbon dioxide emissions, and that is why this graph is plotted as a grey band, rather than a single line – the band represents the range of expected global warming according to lots of different models and calculations. Those different models and calculations all agree pretty well.

Expected warming from CO₂ emissions

Figure 2. Global warming due to total CO₂ emissions with observed data and future predictions for different mitigation strategies

According the graph, when we have emitted 3670 Gt CO₂ (3.67 trillion tonnes), we can expect the global temperature to have risen between ~1.4 and ~3.1 °C (black dotted line shows 3.67 trillion tonnes vs 2 ܄C).

The black ellipse labelled “Observed” plots our actual total CO₂ emissions between 1870 and 2005 against the observed warming (as an average of the years 2000 to 2009 compared to an average of the years 1861 to 1880). As you can see, it fits the predicted warming very well.

The red line shows the cumulative total CO₂ emitted as of today [4], December 23rd 2014 (2150 Gt CO₂) and that, no matter what happens, we are already committed to global warming of between 0.8 and 1.8 °C.

The other ellipses are predictions for the amount of CO₂ we will have emitted in the year 2100 for different CO₂ mitigation strategies, and the amount of global warming we would expect. Figure 3, below, is simplified from Figure SPM5 of the IPCC 2014 report and shows the corresponding CO₂ emission scenarios. The blue scenario (blue ellipse in Figure 2, blue band in Figure 3) represents the likely outcome if we use an aggressive strategy to cut CO₂ emissions and are actually able to completely stop emitting CO₂ and start creating negative emissions – actively taking CO₂ out of the atmosphere faster than nature. The yellow scenario represents a CO₂ reduction strategy that sees us managing to stabilise global CO₂ emission rates at their current levels (i.e. the emission rate doesn’t increase) and then decrease emission rates in about 30 years time. In this scenario, we would still be releasing around 50 Gt CO₂ every year, to start with, and that means we would have spent our CO₂ budget within about 30 years; by 2100 we would have emitted 4-5 trillion tonnes of CO₂ and would see global warming of 2.5 to 3 °C. The orange scenario shows what would happen if we slow the increase in emission rate and stabilise CO₂ emissions in about 3 years time – by 2100 we would have emitted ~ 6 trillion tonnes of CO₂ and would see global temperature rises of 3 – 4 °C. The red scenario represents “business as usual” – what we would expect to happen if we didn’t make any special effort to reduce CO₂ emissions. In this scenario, emission rates would continue to increase over time and we are looking at > 6.5 trillion tonnes of CO₂ emitted with temperature rises of > 4 °C by the year 2100.

Predicted CO₂ emissions

Figure 3: Predicted annual CO₂ emissions over time for different mitigation strategies

In summary, if we want a good chance at avoiding a 2 °C global temperature rise, then we can’t emit more than 3.67 trillion tonnes of CO₂. We have already emitted more than half of that in just the last 145 years. Our global CO₂ emission rates are continuing to rise and if we don’t take action quickly, we will have reached that limit within 30 years.

Notes and references:

[1]  Pacala, S. & Socolow, R. Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies. Science 305, 968–972 (2004).

[2]  Archer, D. Fate of fossil fuel CO₂ in geologic time. Journal of Geophysical Research 110, (2005)

[3] Allen, M. R. et al. Warming caused by cumulative carbon emissions towards the trillionth tonne. Nature 458, 1163–1166 (2009).

[4] Reference [3] talks about “the trillionth tonne” – why are you talking about a budget of 3.67 trillion tonnes?

CO₂ emissions can be described in terms of mass of CO₂, or mass of C (i.e. the mass of carbon in carbon dioxide): The element carbon has an atomic mass of 12 and oxygen has an atomic mass of 16. So carbon dioxide has a molecular mass of 44, which is 3.67 times greater than the mass of carbon. Allen’s paper [3] and the website http://trillionthtonne.org/ both discuss CO₂ emissions in terms of mass of carbon – emitting 1 trillion tonnes of carbon (as carbon dioxide) will produce 2 °C warming. To convert this to mass of CO₂ we just multiply by 3.67. The graphs I used in this blog post used emissions and emission rates for CO₂ rather than just C, so I used CO₂ emissions and emission rates to make things simpler. The 2150 Gt CO₂ I quoted as having been released between 1870 and today was calculated by looking up the real-time cumulative total C-emissions from http://trillionthtonne.org/ (586 Gt) and multiplying it by 3.67.

[5] http://ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_LONGERREPORT.pdf

IPCC 2014 report: Figure 1.1 explained

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.