Effects of COVID lockdowns on climate change: figure descriptions

What does this figure mean?

Here's a figure I showed in the main blog post. Let's inspect each element of it closely.

 

 

The x-axis is a time axis showing the decade covered by the simulation. The y-axis shows temperature change in ℃. These results are from the two-year blip pathway compared to the baseline pathway. For the curious reader, the baseline assumes that all countries will meet their Nationally Determined Contributions (NDCs) by 2030. It is a central estimate of emissions i.e. neither the best-case not the worst-case scenario.

Now note the evolution of each line on the plot. The aerosols, shown in green, first lead to a temperature increase (because they stop shielding the Earth from the Sun's radiation). In the long term, their effect plateaus out because aerosols have a short residence time in the atmosphere. This means that they can not have any long lasting effects because they get removed from the atmosphere. Next, in pink, is tropospheric ozone. Initially, its concentration dips because NOx dips. Nox is an ingredient for ozone creation in the troposphere. Eventually, when aerosol emissions resume, tropospheric ozone is also created again and as a greenhouse gas, starts trapping heat in the Earth's atmosphere again.

 

Next, in orange is CO₂. Now, I'm sure the reader knows why CO₂ (and hence heat trapped by it) dips..but why does it stay that way in the long-term? That is because of the long residence time of CO₂ in the atmosphere. A part of the CO₂ emitted is actually expected to remain in the atmosphere for centuries and perhaps even millennia. That's what makes the climate crisis especially tricky - every day, we emit CO₂ for our convenience but we lock in heat-trapping not just for the next few years, but for many many generations to come.

Finally, we come to the total in blue. Short-term warming, and some long-term cooling. Much Ado...for nothing?

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Source: IPCC AR5 (Figure 7.18)

 
The x-axis shows various aerosols. From left to right: Sulphate (in red), Black Carbon from Fossil Fuel and Biofuel (in black), Primary Organic Aerosol from Fossil Fuel and Biofuel (in green), Biomass Burning aerosols (in dark green), Secondary Organic Aerosols (in olive), Nitrate (in orange), Mineral (in grey) and lastly, there's a Total (in blue). These show the main aerosol types that affect climate.

The y-axis shows something called RFari and its units are in W/m². What is this?

RF stands for Radiative Forcing and its units are always in W/m². This is a term you will come across a lot in climate change literature and it is important to get at least a general understanding of it. In very simple terms, it means "contribution towards climate change". For a deeper explanation, go here. When RF is positive, it means that thing or process contributes to trapping energy in the Earth's system - i.e. it contributes to climate change. When RF is negative (see the "Total" in this figure, for example), it means that thing or process does the opposite - and hence reduces climate change.

You may now wonder - what does the "ari" mean in RFari? Aerosols have two main effects on the climate. One is from the aerosol-radiation interactions (ari) and the other is from the aerosol-cloud interactions (aci). In this figure, only the RF resulting from ari have been shown. [It is very, very difficult to calculate RFaci - this is still a challenge in climate science.]

Now let's look at the aerosol type Sulphate, shown in red. There are two bars. This is done to show different estimates and their uncertainties. The left one - a hatched whisker plot - shows modelling results. The right one - the solid bar - shows the best estimate and 90% uncertainty range, according to the IPCC. Let's look at this best estimate. For Sulphate, the RFari turns out to be -0.4 ± 0.2 W/m². The negative RFari means that sulphate aerosols reduce climate change through their aerosol-radiation interactions (ari).

Compare this with Black Carbon, shown in black. The RFari is 0.4 ± 0.4 W/m². Clearly, both the estimate and the uncertainty are higher in this case. The positive RFari means that black carbon traps heat and contributes to climate change through its aerosol-radiation interactions.

Finally check out the last bar, the Total, shown in blue. It shows that in total, through their aerosol-radiation interactions, all these aerosol types combine to generally reduce climate change. Good news, eh?

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Source

This is a very interesting figure. It shows the effects volcanoes have on global temperatures.

The x-axis is a time axis from 1750-2000. The y-axis shows the average earth land temperature in ℃. The dark black line shows the smoothed values, while yearly temperatures are also shown in fainter black. Several different eruption events are marked in blue. Note how every major eruption has led to downward spike in the global temperature. This is because when volcanoes erupt, they release large amounts of aerosols which shield the Earth from the Sun's radiation. This is the aerosol-radiation interaction we discussed earlier and this also confirms the negative value we saw in the Total in the previous plot.

This figure seems to be from BerkeleyEarth.org but I picked it up from this Mongabay article. It's an amusing article.