Dear Alastair,

thank you for the question. The Heinrich events are quite a compex and challenging subject of palaeoclimatology. Effects of any particular Heinrich event differ substantially across time and space. So, could you perhaps elaborate a bit further on what precisely you mean by "cause warming"? Which warming at which place and time exactly are you referring to there?

Here's also quite a recent publication you may find interesting:

Ziemen, F., Kapsch, M.-L., Klockmann, M., & Mikolajewicz, U. (2019). Heinrich events show two-stage climate response in transient glacial simulations. Climate of the Past, 15, 153-168. doi:10.5194/cp-15-153-2019.

Best,

Julius

Hi Julius,

The abrupt warming I am particularly interested in is the one which happened at the end of the Younger Dryas, described in the book: Alley, R. B. (2000) "The two-mile time machine", Princeton, New Jersey, USA, Princeton University Press. Wally Broecker claimed that the Younger Dryas was H0, but why do Heinrich events cause rapid warming?

The other rapid warming which I am looking for a cause is the one that happened at the start of the Bolling-Allerod inter-stadial. Was that the end of H1 or is it unrelated to Heinrich events?

Thanks for the Zieman et al. reference. I have not gotten around to reading it yet, but I will soon.

Cheers, Alastair.

Dear Alastair,

many thanks for the follow-up. Yes, the Younger Dryas is sometimes referred to as H0. However, this does not mean that this "Heinrich" (if you want to classify it like this) event CAUSED the abrupt warming AFTER the Younger Dryas. My understanding is that this abrupt warming signals the natural - and final - departure from glacial to interglacial conditions.

I would put forward a similar line of argument for the earlier case you mention: H1 was a significant late glacial cooling event. However, it did not CAUSE the Boelling-Alleroed interstadial. Rather, the Boelling-Alleroed was a first departure from glacial conditions, later to be - temporarily - reversed by the Younger Dryas (H0, if you will).

That's just my reading. Others may interpret this differently.

All the best,

Julius

Hi again Julius,

I my problem is not what caused those two events, but rather what caused them to be abrupt? And what causes Heinrich events to end with an abrupt warming?

The Zieman et al paper describes Heinrich events as two stage processes but both produce cooling. It seems they do not have an explanation of the rapid warmings which end Heinrich events.

Cheers, Alastair.

Hi again Alastair,

I'll try to rephrase my explanation a bit - hopefully making it clearer: Soon after the LGM had ended, the Earth's orbital parameters were due for the onset of the next interglacial. All other parameters were "GO" as well, as it were, so Earth was bound for another interglacial. As you know, these periods often start with an abrupt warming (as opposed to the more gradual descent into ice age conditions). As always, many different drivers and fluctuations are at work at the same time - some of them working in the opposite (= cooling) direction. Two of those were the Heinrich events and the sequence of Dryas events. However, they were not "strong" enough to pull Earth away from the impending interglacial conditions. You can think of it as an enromous amount of potential energy being "stored" by the Milankovic cycles. Heinrich and Dryas delayed the onset of warm, interglacial conditions for a while, but couldn't "prevail", so to speak. So the potential energy of the orbital parameters (Milankovic) eventually pushed Earth into our present interglacial, this "potential energy" being released in a the abrupt warming following the Younger Dryas (as well as, to a smaller degree, H1). You may think of these systems dynamics - the other way around - also for the present day conditions: As you know, a new ice age would orbitally be long overdue now. However, anthropogenic GHG (especially CO2) emissions have pushed the planet across the critical threshold, so that a new glacial inception is very unlikely for the next few hundred years or so (unless we manage to pull out the GHGs from the atmosphere artifically...). Nevertheless, the powerful orbital/astronomical (Milankovic) cycles are still there. Their "potential energy", so to speak, hasn't gone away, but is just masked by - temporarily (?) - more powerful intervening anthropogenic forcing. You may metaphorically also think of it as a bow that is drawn, but which needs your hand to let go to eventual release its enormous potential energy. Some other forces (e.g., another person's hands = Heinrich/Dryas events) may hold back the eventual release of this enormous amount of energy.

Hope this helps a bit.

Best,

Julius

Sudden changes in the distribution of heat around the globe are common in the past climatic record. The present unstable climate that began in 2018 appears to be due to a sudden increase in the thermal gradient between the equatorial regions and the arctic air mass. This is being redressed by increased heat movement from the tropical regions to the Arctic in the form of stronger subtropical air masses bottling up the Arctic air mass in winter as well as increased Atlantic Hurricanes that are larger and last for a longer period of activity. Similarly, there are increases to the monsoon activity in the Pacific and Indian Oceans. Deserts such as the Sahara are extending their area of influence, and increased desertification is occurring around the Sahel in Africa as well as around the Mediterranean Sea. In addition, the Arctic Air mass has been bottled up in northern Canada by the strengthened Subtropical air mass, but it eventually breaks out in a very strong cold air flow south to the Caribbean area and northern Mexico. As a result, there are fewer but stronger out flows of arctic air each winter since the winter of 2020-2021 reaching south to the Caribbean Sea and Northern Mexico.

During 2018, there was a spectacular cooling of the main Antarctic Continent resulting in a new cold air temperature world record of -98 degrees C. This is approximately 12-14 degrees C colder than the previous record and implies that a large mass of air travelled north towards the Equatorial regions. It would be equal to 5-6 times the air temperature change that the recent climate conference in Glasgow was discussing, and appears to be about the right magnitude to cause the abrupt climate change discussed above. For more details, see doi: 10.5772/interchopen.101414.

Stuart.

There is one theory that's had serious consideration since 2007 and here is more recent review:

Article The Younger Dryas impact hypothesis: Review of the impact evidence

Abstract

Firestone et al., 2007, PNAS 104(41): 16,016–16,021, proposed that a major cosmic impact, circa 10,835 cal. BCE, triggered the Younger Dryas (YD) climate shift along with changes in human cultures and megafaunal extinctions. Fourteen years after this initial work the overwhelming consensus of research undertaken by many independent groups, reviewed here, suggests their claims of a major cosmic impact at this time should be accepted. Evidence is mainly in the form of geochemical signals at what is known as the YD boundary found across at least four continents, especially North America and Greenland, such as excess platinum, quench-melted materials, and nanodiamonds. Their other claims are not yet confirmed, but the scale of the event, including extensive wildfires, and its very close timing with the onset of dramatic YD cooling suggest they are plausible and should be researched further. Notably, arguments by a small cohort of researchers against their claims of a major impact are, in general, poorly constructed, and under close scrutiny most of their evidence can actually be interpreted as supporting the impact hypothesis.