CO2 footprint per bitcoin transaction Out of personal interest I wanted to find out which carbon footprint one bitcoin transaction produces. Following I will present my calculation. The result surprises me and my question is Is this realistic? Did I make mistake?

Variables


Transaction throughput: $\approx 4\frac{tx}{s}$  source
Bitcoin energy consumption $\approx 8.00GW$ source
Carbon emission per $\approx \frac{0.5kg}{kWh}$ CO2 germany (Not sure if China is lower or higher here, since mining mostly happens in China)


Calculations
Transactions per hour: $4\frac{tx}{s} = 60*4\frac{tx}{m} = 60*60*4\frac{tx}{h} = 14.400\frac{tx}{h}$
Energy Consumption per hour: $8.00GWh = 8 * 10^6kWh$
Energy Consumption per transaction $\frac{8 * 10^6kWh}{14.400tx} \approx 555\frac{kWh}{tx}$
Carbon footprint per transaction $555\frac{kWh}{tx} * \frac{0.5kg}{kWh} \approx 277\frac{kg}{tx}$

If the whole energy used for bitcoin was produced in germany one bitcoin transaction would have a carbon footprint of 277kg. If we take Finland for example with more reneweable energies ($\frac{0.113kg}{kWh}$) it would be 77kg CO2. 

For reference: A single flight form Hamburg to Munich (600km, aprox. 1h) is 161kg myclimate. The compensation of 277kg CO2 is roughly $6.50 with atmosfair.
I did some napkin math based on your data. At 4 tx/s and 8GW, that is 2GWs per transaction. 0.5kg/kWh is around 139kg/GWs. Thus each transaction is around 278kg. Looks like we reached the same answer. 

But is this the carbon footprint of one Bitcoin transaction? To put it differently, if there was a global carbon tax, would it makes sense to charge for 278kg per transaction? Probably not. In fact, almost certainly most of the tax would just apply to the electricity bill of Bitcoin miners, which is split between hashing for the block reward and processing transactions.

There are roughly 2000 transactions per block. Each transaction costs roughly \$1. The block reward is 12.5 Bitcoins as of now. At \$8200 per Bitcoin, roughly $\$2000/(12.5\space\text{BTC} \times \$8200/\text{BTC} + \$2000)=0.0191$ or 2% of a miner's electricity bill goes to transaction processing. So perhaps it makes more sense to say that each transaction costs around $278\space\text{kg} \times 0.02=5.56$kg of carbon.

So what about the rest of the carbon? In some sense, the main job that PoW does is preserving the integrity of existing token balances. The block reward is seniorage. This a similar principle to how central banks print more money and thus reduce the value of existing bank notes and coins. As of now there are close to 18 million Bitcoin in existence. Continuing with your carbon emission numbers, each Bitcoin holder that isn't transacting is then responsible for $((8/(18 \times 10^6)) \times 139) \times 0.98=0.000060542$kg of carbon per second per Bitcoin or 5.23kg of carbon per day per Bitcoin.