How much can El Niño be blamed for the jump in CO2?

Something unusual happened in April this year. Between March and April there was a record month to month jump in atmospheric CO2 recorded at Mauna Loa. It went up by 2.71 ppmv. That's 0.6 ppmv higher than the previous highest month to month jump (2.11 ppmv) back in April 2004. However it's not wise to focus on month to month variations. For example, March could have been lower than expected, which would make the jump seem larger than it should be. (It wasn't.) This article explores some of what causes atmospheric CO2 to go up and down. It's not the answer to everything, however I learnt a lot doing the research and I have some unanswered questions too (like a quantitative answer to the title of this article). (Let me know if you see any mistakes I may have made.)

The chart below shows the month by month increase in atmospheric CO2 since the late 1950s. Hover over the chart for the values. The most recent is April at 407.57 ppmv.

Figure 1 | Monthly CO2 at Mauna Loa. Data source: SCRIPPS CO2 Program

Below is the same plot but starting from January 1995, with a linear trend line added. Yes, I know CO2 is not rising in a linear fashion. However a linear trend line helps to see what is happening over short-ish periods. From April 1995 to March 2016, CO2 has been increasing at a rate of  2.0 ppmv a year - just using a linear trend. I've used that period so that you can compare this past El Nino with the 1997/98 El Nino. (Hover over or click on the chart to see the numbers for each month.) [Note: prompted by a reader who reminded me of end point distortions, I amended the trend line to cover a full 12 month period from April to March. Sou.]

Figure 2 | Monthly CO2 at Mauna Loa from January 1995 to April 2016. Data source: SCRIPPS CO2 Program

April CO2 was way above the linear trend

To see if April this year really was unusual, I've plotted the difference between the observed CO2 and the linear trend from April 1995 to March 2016. That is, the observed atmospheric CO2 less the trend value for each month.

Figure 3 | Monthly CO2 at Mauna Loa less the linear trend. from January 1995 to April 2016. Data source: SCRIPPS CO2 Program

As you can see, in April this year, the difference between the observed CO2 and the trend line was greater than in any other month since January 1995. In April, atmospheric CO2 was 5.93 ppmv higher than the April 1995-March 2016 linear trend. On average, April is 2.63 ppmv above the trend (not including April 2016). The previous highest April was in April 1995, at 3.95 ppmv above the trendline for that month. In 1998, the year of the last big El Nino, April was 3.09 ppmv above the linear trend.

Since the long term trend is not linear, the rate of increase is itself increasing over time, below is a plot of the measured CO2 less the trend line from April 2001 to March 2016. The linear trend for that shorter period is higher at 2.10 ppmv a year. April this year was 5.33 ppmv above the linear trend.

Figure 4 | Monthly CO2 at Mauna Loa less the linear trend from January 2001. Data source: SCRIPPS CO2 Program

So it looks as if there was indeed unusually high CO2 in April this year. It's probably too soon to tell whether that is just a blip or whether it signals a shift. It could be that the sinks have had more than they can handle or it might not signify anything at all. Or it could be that there was a big jump in emissions recently. Annual emissions data shows a tapering off, but I don't have monthly data.

Was it just El Niño?

In an article for Associated Press, Seth Borenstein quoted NOAA scientists Jim Butler and Pieter Tans, and SCRIPPS scientist Ralph Keeling, saying CO2 was boosted by El Niño. What Pieter Tans said was:

In many places El Nino means more drought and fires, which release more carbon dioxide into the air and decreases carbon dioxide sucked up by plants.

That makes sense though it doesn't explain the extent of the increase. In the previous HW article, I wrote about how El Niño's mean more drier weather on land, and as you know El Niño years are hotter. Hotter plus drier means more droughts and fires. The land affected becomes a net source of atmospheric CO2. Thing is, this didn't happen in 1998 when there was also a very strong El Nino.

A CO2 forecast from scientists in Nature Climate Change

There's also a new paper out in Nature Climate Change, by Richard A. Betts, Chris D. Jones, Jeff R. Knight, Ralph F. Keeling and John J. Kennedy. They provided a forecast of the growth in atmospheric CO2 for 2016. The paper is very current. Although the forecast is based on data to October 2015, it includes mention of the latest observation in April 2016. The authors confirmed the reason CO2 is above what it would probably have been had there not been an El Nino, writing:

The long-term rise in atmospheric CO2 concentration, approximately 2.1 ppm yr⁻¹ over the past decade, is caused by anthropogenic emissions arising from fossil fuel burning, deforestation and cement production. The annual growth rate, however, varies considerably as a result of climate variability affecting the relative strength of land and ocean carbon sources and sinks. The annual growth rate measured at Mauna Loa, Hawaii is correlated with the El Niño– Southern Oscillation (ENSO), with more rapid growth associated with El Niño events through drying of tropical land regions and forest fires. 

Although the authors weren't analysing the overall contributions to atmospheric CO2, in the paper they did provide some detail of what would have contributed to it recently, including:

• anomalously dry tropical land regions
• human-caused fires over large areas in Indonesia.

Their forecast is that between 2015 and 2016, CO3 will increase by 3.15 ± 0.53 ppm, giving an annual mean CO2 concentration for 2016 of 404.45 ± 0.53 ppm. They wrote that the forecast would be a success if the 2016 annual mean CO2 concentration is between 403.92 and 404.98 ppm. Anything higher would suggest unexpected non-linearities, said the authors. They also added a paragraph about the April 2016 values:

Our method predicted an annual maximum monthly mean of 407.57 ± 0.53 ppm in May; and 406.70 ± 0.53 ppm for April. The observed CO2 concentration for April was 407.57 ppm so the method slightly underestimated the concentration for that month, although this was not the case in the first three months of the year (Fig. 3b). The concentration in April 2015 was 403.45 ppm, but although the 4.12 ppm increase over the 12 months to April 2016 is a clear indicator of a large annual growth rate this year, it may not be representative of the precise annual mean value. Data available at the time of going to press indicate that the mean concentration in May 2016 is similar to that in April. 

They wrote an estimate of by how much El Nino influenced CO2:

For comparison, persistence of the last decade’s mean growth rate of 2.1 ± 0.1 ppm yr−1 from the 2015 value of 400.9 ppm would have suggested a concentration of 402.9–403.1 ppm. The processes related to the El Niño are therefore estimated to contribute approximately an additional 1 ppm to this year’s CO2 growth rate. 

 While I agree that the linear trend since 2000 has been around 2.1 ppm a year, because CO2 is rising faster and faster, it might not be quite right to attribute all the difference to El Nino. Some of it would just be that the rate of increase continues to increase. Still, over that fairly short period of sixteen years it's probably close enough.

Sources and sinks and ENSO

The change in atmospheric CO2 can be expressed as the following simple equation:

ΔCO2 (atmosphere) = CO2 (human emissions) + CO2 (natural sources) - CO2 (natural sinks)

The reason atmospheric CO2 is increasing is because we are adding it to the atmosphere. The CO2 sinks can't keep up with the increase. If it weren't for that, the amount emitted by the natural sources each year would be balanced by the amount absorbed by the natural sinks.

In the short term, natural sources of CO2 each year are plants and other organisms on land and in the ocean, and parts of the ocean itself, where there is an exchange of CO2 with the air. Natural sinks are the same.

You might have thought the recent increase from El Nino was because the sea surface has been hotter in the Pacific. That's what I thought at first, but I was wrong. In fact, in the Pacific, the Nino-region seas outgas less CO2 in an El Nino, despite being hotter. As the scientists I referred to above said, the reason El Nino years tend to have higher CO2 increase is because the land is a bigger net source, and that's because plants aren't absorbing as much, and because fires and drought are more prevalent in an El Nino year. (See the recent HW article about how El Nino years are drier overall.)

I had to do a lot of digging to work out just what happens over time. Unfortunately, the OCO-2 project isn't releasing much that I can use. There is a video, but it only goes from September 2014 to September 2015 so you can't compare the same months in successive years:

Here is a snapshot from the video of the period from 14 May to 29 May last year, at the start of the El Nino. You can see all the CO2 coming out of the northern hemisphere. May is usually the month when atmospheric CO2 is at its highest for the year, just before plants start being a net sink again. (It's at its lowest in September-October each year.)

In the map above the red splotches are where CO2 is at its highest concentration and the blue splotches are where it's lowest. There's a difference of no more than about 10 ppmv (and probably less) because CO2 is very well mixed in the atmosphere.

The air-sea CO2 flux and ENSO

In the equatorial Pacific ocean, the main impact of ENSO events (or lack of them) is through upwelling or its suppression. Upwelling of water from the deep in the eastern Pacific brings up carbon rich waters from the depths to the surface. La Nina has most upwelling, El Nino suppresses it. In a La Nina, stronger trade winds blow east to west, which results in upwelling along the coast of South America. Nutrients and dissolved inorganic carbon (DIC) are lifted to the surface, which means more CO2-enriched water at the surface, and more CO2 is released to the air. In El Nino, this is suppressed as the warmer water from the west spreads out to the east.

Xiujun Wang and colleagues wrote a paper last year for the AGU journal Global Biogeochemical Cycles that gives a lot of information. For example:

It is well known that the surface pCO2 variation in the tropical upwelling regions is mainly determined by SST [sea surface temperature] and dissolved inorganic carbon (DIC), but DIC and SST have opposite effects, i.e., increasing and decreasing pCO2 during strong upwelling, respectively. Thus, the net effect of DIC and SST on pCO2 may vary over space and time. 

I don't know if the PDO phase change will make a difference. There was a 2003 paper in Science by Taro Takahashi et al that said it does. However from where we are now, it seems to me they got the phase shift wrong so I don't know the extent to which their results would hold today. Anyway, below is a chart to 2015 showing the Pacific Decadal Oscillation, with the shift to the cool phase in the late 1990s and the possible shift back to the warm phase in 2014. If anything, the shift to the warm phase might act to inhibit CO2 in the equatorial Pacific but enhance it in the world as a whole (more droughts, fires etc). The net effect should be faster accumulation of CO2 in the atmosphere I think.

There have been studies looking at the contribution of the other ocean basins as well, but I won't go into them here. This article is quite long as it is.

The air-land CO2 flux

A short while ago there was a paper by Wolfgang Buermann et al. published in the EGU journal Biogeosciences, where the authors wrote of a sudden and sustained increase in the land CO2 sink in the early 1980s. They said that without that, atmospheric CO2 would be 12 ppm higher than it is now. The authors wrote:

The world’s ocean and land ecosystems act as sinks for anthropogenic CO2, and over the last half century their combined sink strength grew steadily with increasing CO2 emissions. Recent analyses of the global carbon budget, however, have uncovered an abrupt, substantial ( 1 PgC /yr and sustained increase in the land sink in the late 1980s whose origin remains unclear. In the absence of this prominent shift in the land sink, increases in atmospheric CO2 concentrations since the late 1980s would have been  30% larger than observed (or  12 ppm above current levels). 

The authors used a model to determine what caused the change. First some definitions:

• Net primary production (NPP) is "the rate at which all the plants in an ecosystem produce net useful chemical energy; it is equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (GPP) and the rate at which they use some of that energy during respiration". 
• Heterotrophic respiration (Rh) is the release of CO2 during the process of decomposition of organic matter in the soil by soil animals, fungi, and other decomposer organisms.
• Net ecosystem production (NEP) is estimated as NPP – Rh)

The authors wrote:

We find that net primary production significantly increased in the late 1980s (more so than heterotrophic respiration), consistent with the inferred increase in the global land sink, and that large-scale climate anomalies are responsible for this shift. We identify two key regions in which climatic constraints on plant growth have eased: northern Eurasia experienced warming, and northern Africa received increased precipitation. Whether these changes in continental climates are connected is uncertain, but North Atlantic climate variability is important. Our findings suggest that improved understanding of climate variability in the North Atlantic may be essential for more credible projections of the land sink under climate change. 

The paper is interesting and not to difficult to follow. It discusses climate shifts in northern Eurasia and north Africa in particular. It also discusses other aspects such as wildfire, and more. The conclusion gives food for thought as well, saying that a shift back to the negative phase of the Arctic/North Atlantic oscillations could reduce carbon uptake by land-based plants. That could mean that atmospheric CO2 would rise more quickly again. They wrote:

Our results point to a mechanism whereby North Atlantic climate variability modulates the global terrestrial carbon cycle. New research suggests that a large portion of the variability in the North Atlantic may be externally forced by anthropogenic aerosols (Booth et al., 2012) and the pronounced warming trend in the Arctic regions, known as Arctic amplification (Cohen et al., 2014). Arctic amplification specifically is thought to intensify under climate change (Deser et al., 2010), and this may drive the AO/NAO more into their respective negative phases (Cohen et al., 2014), which, based on our results, would substantially reduce carbon uptake by terrestrial plants and weaken the land carbon sink. This illustrates the pressing need for improved knowledge of North Atlantic climate variability and associated forcing mechanisms in order to more credibly project the evolution of the land carbon sink and carbon cycle climate feedbacks under climate change. 

CO2 and ENSO from climate modeling

In another recent paper, this time by Jin-Soo Kim et al in the AMS Journal of Climate, authors looked at CMIP5 earth system models to better understand the ENSO/CO2 relationship. In the abstract they wrote how climate models may overestimate the amount of carbon absorbed through plant growth, and underestimate the amount added to the air through heterotrophic respiration and natural fires. The abstract (my paras):

Better understanding of factors that control the global carbon cycle could increase confidence in climate projections. Previous studies found good correlation between the growth rate of atmospheric CO2 concentration and the El Niño-Southern Oscillation (ENSO). Growth rate of atmospheric CO2 increases during El Niño but decreases during La Niña. In this study, long-term simulations of the Earth System Models (ESMs) in the Coupled Model Intercomparison Project Phase 5 archive were used to examine the interannual carbon flux variability associated with ENSO.

The ESMs simulate the relationship reasonably well with a delay of several months between ENSO and the changes in atmospheric CO2.

The increase in atmospheric CO2 associated with El Niño is mostly caused by decreasing Net Primary Production (NPP) in the ESMs. It is suggested that NPP anomalies over South Asia are at their maxima during boreal spring; therefore, the increase in CO2 concentration lags four to five months behind the peak phase of El Niño. The decrease in NPP during El Niño may be caused by decreased precipitation and increased temperature over tropical regions. Furthermore, systematic errors may exist in the ESM-simulated temperature responses to ENSO phases over tropical land areas, and these errors may lead to overestimation of ENSO-related NPP anomalies.

In contrast, carbon fluxes from heterotrophic respiration and natural fires are likely underestimated in the ESMs compared with offline model results and observational estimates, respectively. These uncertainties should be considered in long-term projections that include climate–carbon feedbacks.

From the Deniosphere

There have been a couple of articles in the deniosphere about the jump in CO2. Jo Nova decided it means that burning fossil fuels doesn't add CO2 to the air. She didn't say that outright, but that's what she implied. That's too stupid to bother commenting about. If you want to know more about the basics, try this article about Murry Salby's equally nutty ideas.

Anthony Watts at WUWT just copied and pasted the press release about the new paper by Richard Betts and co. (archived here).  The comments were pretty much what you'd expect from his conspiracy theorists:

MarkW still doesn't believe that CO2 stays in the air for a very very long time:

June 13, 2016 at 12:12 pm
They are still trying to defend a long residency time for CO2?

Francisco Fernandez gets a bit confused in the translation. It's not human emissions that are getting a boost, it's the atmospheric carbon dioxide concentration that is getting a boost. When there are more dry spells then plants won't grow as much so they'll use up less CO2, and more fires burn, which adds CO2 to the air:

June 13, 2016 at 12:13 pm
Might be my English and the wording, but how can El Niño boots anthropogenic emissions? It can boost natural, but not anthropogenic.
Does this mean that our measly 0.12% attribution to the atmosphere is now even smaller? Go figure 

Glen Haas thinks it will help agriculture. Maybe it will and maybe it won't, it depends on where the agriculture is. Remember, one of the main reasons there's more in the air is because it's been drier and plants haven't taken up as much CO2 (which means they haven't grown as much). Plus there've been more fires which isn't good for plants.

June 13, 2016 at 5:55 pm
Many of us caught this. However, the increase in CO2 will be great for agriculture around the world. We need to count our blessings.

Tom Halla is a greenhouse effect denier who wrongly thinks that the CO2/temperature relationship only goes one way. He's wrong. Increasing CO2 makes the world hotter through the greenhouse effect. There'd be some positive feedback, when global temperatures warm the oceans they release more CO2 except for times like now when the partial pressure of CO2 is so high that it means that the oceans are still absorbing more CO2 than they are emitting.

June 13, 2016 at 12:23 pm
It is also consistent with the assertion that temperature drives CO2 levels, not the reverse.

Kiwikid spouts some pseudo-science nonsense and decides he or she knows more about the subject than climate scientists, which isn't unusual for the Dunning-Kruger set at WUWT:

June 13, 2016 at 12:36 pm
The conclusions of the article, that the CO2 has risen this year because of biosphere release adding to human emissions is not correct.

It is the troposphere atmospheric capacity increase in the mid to high NH latitudes, created by the El Nino heat transport northward that allowed the conditions for tropospheric retention of CO2. It is the atmospheric temperature during winter and spring in the NH mid to high latitudes that is the only controlling factor.

The controlling factors are many and various. For the short carbon cycle they include human emissions, land use changes, cement production, atmospheric temperature, partial pressure of CO2, ocean uptake and outgassing, plant uptake and decay, and much more.

James Allison asks about what is "officially accepted".  The most "official" is the COP21 agreement, which is no more than to cause a 2C rise in temperature, though if possible only 1.5C rise:

June 13, 2016 at 12:45 pm
Is there an officially accepted level of CO2 that humans release into the atmosphere or is it entirely dependent upon which scientist happens to be talking?

I cannot be bothered trying to work out what is going through afonzarelli's head:

June 13, 2016 at 5:23 pm
The carbon growth rate has been tracking with temperature since the inception of the MLO data set over half a century ago. If we see cooling in the upcoming years, hopefully that will destroy the myth that plants are taking up extra co2 as the planet greens. Cooler temps mean greater uptake, while warmer temps mean lesser uptake. Hopefully, too, a nice prolonged cooling spell will send ferdinand out to pasture as well… (☺) 

joelobryan is one of the wackier deniers at WUWT. He is hoping that what he thinks is a two hundred year old "climate hoax" conspiracy will unravel soon:

June 13, 2016 at 5:08 pm
Adjustments will made assuming the political regimes that enables the scam remain inplace. Even Stalin’s Lysenkoism came to an end when his handpicked predecessor fell from power.
Eventually the scam will collapse. The big guys at these government-run climate centres will simply retire and sail away in their pension lifeboats to be forgotten. The carnage will be the young and mid-career scientists left holding the shit bags they filled. 

FJ Shepherd is probably right. There could well be a limit beyond which the sinks take up a lesser proportion of emissions:

June 13, 2016 at 1:58 pm
Has everyone missed the “it’s worse than we thought scenario”? As global temperatures rise, natural sinks for CO2 like the ocean, will absorb less CO2, and this will cause more atmospheric CO2 and this will turn into a vicious cycle that will escalate global warming to such an extent… well, that it may become so hot that the oceans could boil. Be afraid; be very afraid.

Peta in Cumbria needs to learn some basic botany and biology (such as plant respiration) as well as how to read CO2 data. Some climate science wouldn't hurt either:

June 13, 2016 at 3:07 pm
Its all so ar$e about t1t its just unbelievable..
So, Richard Betts, explain these things..
Why are stomata on the underside of plant’s leaves – are the plants not perhaps expecting their food to be coming up from under them?
What plants exhale CO2, its beyond crazy for them to do so after they’ve gone to so much trouble collecting it. We’ve seen it written that they ‘need energy’ What! So they can go out chasing woolly mammoths to bring home for the BBQ?
Why did the generally accepted CO2 curve start ramping up at the end of WW2 – what ‘fossil fuel’ event occurred around then – apart from the overnight conversion of munitions factories to agricultural fertilizer factories. What does nitrogen fertilizer actually do – have you any idea
3 questions about why CO2 ramps up in the NH autumn.
1. Is it not because a lot of man-made plants (corn, wheat, rice etc) effectively die and stop taking it up?
2. Is it not because the soil temperature in the NH reaches its annual peak in the autumn and the things creating the CO2, dwelling in the soil and being very temperature sensitive are at their most active?
3. Why do grassland farmers notice a massive surge in grass growth in the autumn, the so-called autumn flush even when they have not spread any fertilizer
If any or all these things are actually happening, does it not explain your El Nino = high temps = high CO2 rise

There are a lot more comments from WUWT's conspiracy theorists. Climate science upsets a lot of people at WUWT. They don't want to believe it therefore it's not so. (I wonder do they react the same way if their credit card statements show they are over-extended?)

References and further reading

Buermann, Wolfgang, Claudie Beaulieu, Bikash Parida, David Medvigy, George J. Collatz, Justin Sheffield, and Jorge L. Sarmiento. "Climate-driven shifts in continental net primary production implicated as a driver of a recent abrupt increase in the land carbon sink." Biogeosciences 13, no. 5 (2016): 1597-1607. doi: 10.5194/bg-13-1597-2016 (open access)

Betts, Richard A., Chris D. Jones, Jeff R. Knight, Ralph F. Keeling & John J. Kennedy. "El Niño and a record CO2 rise." Nature Climate Change (2016) doi:10.1038/nclimate3063

• El Nino drives fastest annual increase on record of carbon dioxide - press release at ScienceDaily.com

Takahashi, Taro, Stewart C. Sutherland, Richard A. Feely, and Catherine E. Cosca. "Decadal variation of the surface water pCO2 in the western and central equatorial Pacific." Science 302, no. 5646 (2003): 852-856. DOI: 10.1126/science.1088570

Kim, Jin-Soo, Jong-Seong Kug, Jin-Ho Yoon, and Su-Jong Jeong. "Increased atmospheric CO2 growth rate during El Niño driven by reduced terrestrial productivity in the CMIP5 ESMs." Journal of Climate 2016 (2016).

Wang, Xiujun, Raghu Murtugudde, Eric Hackert, Jing Wang, and Jim Beauchamp. "Seasonal to decadal variations of sea surface pCO2 and sea‐air CO2 flux in the equatorial oceans over 1984–2013: A basin‐scale comparison of the Pacific and Atlantic Oceans." Global Biogeochemical Cycles 29, no. 5 (2015): 597-609. doi: 10.1002/2014GB005031 (pdf here)