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I am sure there are different methodologies to arrive at such a number. Can someone in layman terms describe some of the most widely used and trusted methods and what data are used? Sorry I am outside the field of climate modeling so the simpler the better.

I can imagine a satellite observation-based model that calculates emissions on a spatial basis but I am not sure if our technology is advanced enough to do that accurately. However, this method would not allow segregation of GHG by source/sectors (electricity generation, cement production, agriculture etc.)

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  • $\begingroup$ Welcome to EarthScience.SE. Emission modeling could be considered as a field of research of its own (at least from my experiance). The emissions are made together from different sources. In German I would say "Man bastelt sie zusammen" :-) . For the CO2 emissions from power production, the emission modelers take the power production/consumption in specific regions, look up the shared of coal and gas power plants, take emission factors for these power plants (CO2 emissions emitted per kWh electricity produced) and calculate the power-production-related CO2 emissions for this region. $\endgroup$ – daniel.neumann Jan 20 at 10:56
  • $\begingroup$ For biomass buring emissions, satellite data (for area and time of fires) combined with emission factors might be used. In general, the emission modelers take basic data of one sector (power consumption, car density, aera of arable land, need for heating, ...), some usage statistics (types of power generation facilities, car usage per capita, ...) and CO2/CH4/N2O/... emission factors. From these information they calculate CO2 (or CO2-equivalent) emissions per sector. The sector emissions are, then, merged to total emissions. $\endgroup$ – daniel.neumann Jan 20 at 11:10
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    $\begingroup$ You might look into Appendix II (p. 1288) of the Working Group III contribution to the IPCC’s Fifth Assessment Report (AR5). Looking at the pictures on pages 1294 to 1297 and scanning a bit through the text might give you some more infos (it is not too technical). $\endgroup$ – daniel.neumann Jan 20 at 11:12
  • $\begingroup$ Thank you so much! $\endgroup$ – RicardoR Jan 21 at 4:58
  • $\begingroup$ Please also note the recent publication Friedlingstein et al. (2019): "Global Carbon Budget 2019". $\endgroup$ – daniel.neumann Feb 5 at 12:48
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Your question is simple enough, but the answer depends on what exactly you're looking for.

Who is emitting where right now?

Real-time global monitoring of greenhouse gas emissions with a high spatial resolution is an emerging technology. We have very useful satellites (see Jean-Marie Privals answer), but they have limitations:

  • All existing public satellites with instruments for high-resolution GHG measurements are in low Earth orbit, so they only see a particular place when they pass over, not all the time. So far only the Geostationary Interferometric Infrared Sounder (GIIRS) on the Chinese FengYung (FY)-4A satellite can in theory monitor CO₂ from a geostationary viewpoint, which allows for "continuous" monitoring (probably meaning hourly; it takes a while to scan the area of interest), but from what I've heard GIIRS is not performing very well. Europe plans to launch the Infra-Red Sounder (IRS) in 2023. Either way, their spatial resolution will be much worse than for the low Earth orbit satellites, because geostationary orbit is so far away.
  • They rely on visible radiation. Although attempts to retrieve from infrared radiation exist, their information content is quite poor (for an Arctic methane example, see Holl et al. (2016)). In Jean-Marie Privals answer you can see an illustration of how reflected sunlight is used as a retrieval.
  • They usually need clear skies. We can't look below the clouds, but even retrieving above the cloud would require a very accurate characterisation of the cloud, and clouds are tricky. So usually we just assume that the GHG concentration is the same with or without clouds, even when it probably isn't.
  • Some private satellites exist with a very high spatial resolution, but I can't find much verifiable information about them. GHGSat claims a spatial resolution of less than 50 metre. Inevitably, that comes at the cost of field of view (12×12 km² claimed), it will only view a spot when actively pointing, so although it may be able to point anywhere on Earth, it will only view very specific areas and is in that sense not global. It appears many similar commercial instruments are planned in the near future.

I can imagine a satellite observation-based model that calculates emissions on a spatial basis but I am not sure if our technology is advanced enough to do that accurately. However, this method would not allow segregation of GHG by source/sectors (electricity generation, cement production, agriculture etc.)

The spatial resolution of about 2 km may be good enough for that, unless the electricity plant is next to the cement producer, the emissions occur at night, the factory is switched off when the satellite happens to pass over, or it's cloudy (there are attempts to retrieve in the presence of clouds, but it's harder).

So while satellites are certainly very useful in GHG monitoring, it's difficult to get everything from satellites alone.

Where were emissions last month?

We can average daytime GHG contentration measurements over the period of a month. Combined with chemistry and circulation models, we can then try to estimate in what regions of the Earth those emissions may have occurred, but not with a precision high enough to tell "electricity or cement". This is an average of CO₂ measurements for July 2009:

July 2009 CO₂

The limitations are less serious now: the satellite has multiple attempts to capture a particular scene, and will usually see at least one clear-sky overpass per month, probably multiple. In the image above, there was probably also some form of data fusion to combine with other sources or fill gaps using neighbouring pixels. The longer the time period we average over, the smoother the distribution will look.

What were global emissions last year?

However, if you are looking for global emissions averaged over a long time period, we can make use of the observation that CO₂, and to a lesser degree CH₄, is a well-mixed gas. Here, well-mixed means that it stays long enough in the atmosphere to reach pretty much everywhere given enough time. That means that ultimately, it doesn't matter where you emit. That's why "global" CO₂ concentrations may be measured at Mauna Kea (Hawaii, USA), even though this is far away from any emissions. However, that also means that it doesn't tell us whether the CO₂ was emitted in India, Italy, or Idaho. And much of the emitted CO₂ gets absorbed by the oceans, so the delta between this year and last is not enough to determine global emissions.

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    $\begingroup$ About spatial resolution: I just found out there is another satellite called GHGSat-D (ghgsat.com/who-we-are/our-satellites/claire) which claims to have a 50m pixel. They have some pretty cool images of CH$_4$ plumes above coal mines or hydroelectric dams (see their "Case studies" section). Unfortunately it's private, so the data is not available... $\endgroup$ – Jean-Marie Prival Jan 20 at 15:10
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    $\begingroup$ @Jean-MariePrival Huh, interesting. I've added a bullet point. The're not even listed in WMO Oscar, which many commercial Earth observation satellites are... $\endgroup$ – gerrit Jan 20 at 15:26
  • $\begingroup$ Another way to measure emissions is to use economic data. Sources like the CIA World Factbook cia.gov/library/publications/the-world-factbook give figures for fossil fuel production. Assume that what's dug up or pumped up gets burned fairly soon, do a bit of chemistry, and you get a reasonable ballpark figure. $\endgroup$ – jamesqf Jan 20 at 18:39
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    $\begingroup$ Interesting, I didn't know these satellites operate by visible lights. I guess there are a lot of interpolations going on! You said night emission is a problem, not sure if there is any diurnal variation of GHG emission that they need to account for before just "filling the gaps"? $\endgroup$ – RicardoR Jan 21 at 5:00
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    $\begingroup$ @RicardoR Not really, because any CO₂ emitted at night is still going to be there the following day (it's a long lived gas, staying in the atmosphere for hundreds of years). It's just harder to attribute who emitted it, because it will have dispersed. Moonlight or city lights could work as well in principle, but the instrument would need to be very sensitive and the signal to noise ratio would be much poorer. $\endgroup$ – gerrit Jan 21 at 7:45
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I can imagine a satellite observation-based model that calculates emissions on a spatial basis but I am not sure if our technology is advanced enough to do that accurately.

It is. The first satellite designed to measure GHG is GOSAT, from the Japanese space agency, launched in 2009 and still active today. It was followed by Nasa's OCO-2 in 2014. GOSAT measures CO$_2$ and CH$_4$, while OCO-2 measures only CO$_2$. There is also an OCO-3, which has been sent to the ISS last year, but I'm not sure if it's already active.

So, how does it work? Here is an image from the OCO-3 mission's website, labelled "Artist interpretation of OCO-3 measurement".

Artist interpretation of OCO-3 measurement

OCO-2 mission's website explains the basic principle better than I could do, so I will just copy an excerpt here:

To get the representative values of Xco$_2$, or the amount of CO$_2$ in the measured space, the OCO-2 instrument will measure at a given location, the intensity of reflected sunlight off the Earth's surface at specific wavelengths. Gas molecules in the atmosphere absorb the sunlight at specific wavelengths. So when light passes through the Earth's atmosphere, the gases that are present leave a distinguishing fingerprint that can be captured. The OCO-2 spectrometers, working like cameras, will detect these molecular fingerprints. Then the absorption levels shown in these spectra, like a captured image, will tell us how many molecules were in the region where the instrument measured.

There are also ground-based measurements, they are more precise but punctual, while satellite measurements have a global coverage. Ground-based measurements are actually used to calibrate the satellites. Also, ground-based measurements give concentrations at the surface, while satellite measurements give "column-averaged concentrations" through the atmosphere and are not able (yet) to do vertical profiles, i.e. to know at what altitude the gas contributing to the signal is located.

If you want to dive more in-depth into this, there is a nice "Guidebook on the use of satellite greenhouse gases observation data..." (Matsunaga & Maksyutov, 2018).

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    $\begingroup$ The information content in night-time satellite-based methane retrievals is very poor, in particular in the far north. See this article I did in 2016. $\endgroup$ – gerrit Jan 20 at 14:09
  • $\begingroup$ Well, you are certainly more qualified than me to answer, but since the OP asked for "layman terms" I figured I would give it a shot! :) $\endgroup$ – Jean-Marie Prival Jan 20 at 14:28
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    $\begingroup$ Your answer is pretty good, I added mine too. $\endgroup$ – gerrit Jan 20 at 14:30
  • $\begingroup$ Thank you for your answer! $\endgroup$ – RicardoR Jan 21 at 5:01
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Satellite data is relatively new and is likely to be used to compare and refine calculations of emissions by other means rather than used as the principle means for estimating emissions.

Consumption based accounting is the main way we get the declared emissions, including methane, nitrous oxides and carbon monoxide and others as well as CO2, for the world or individual nations under internationally agreed UNFCCC guidelines.

Consumption based emissions are calculated from how much fossil fuels of each type are used and what their emissions factors are - how much fuel of each type, what kind of use for each type and how much emissions for that use. Similarly industrial emissions and products as well as land uses and estimates of how much emissions or take up of CO2 by the different uses is all calculated.

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