Most mornings over Heathrow there is a queue of aircraft holding as they wait for the 0600 curfew to end. At times this builds from around 0545 and often involves 12 or so aircraft. I did it myself for a couple of decades so tried to calculate my "extra" carbon emisions based on average air holding times - I ran out of zeros on my calculator.
This holding is far more damaging than tankering and I did try to avoid it by slowing down en-route but arriving at the back of that queue still brought the dreaded " take up the hold at Lambourne - maintain FL 160 - at least 20 mins delay"..........