“And Saṃvaraṇa, that bull among men with due rites took Tapatī's hand on that mountain-breast which was resorted to by the celestials and the Gandharvas. The royal sage, with the permission of Vasiṣṭha, desired to sport with his wife on that mountain. And the king caused Vasishtha, to be proclaimed his regent in his capital and kingdom, in the woods and gardens. And bidding farewell unto the monarch, Vasiṣṭha left him and went away. Saṃvaraṇa, who sported on that mountain like a celestial, sported with his wife in the woods and the under-woods on that mountain for twelve full years. And, O best of the Bhāratas, the god of a thousand eyes poured no rain for twelve years on the capital and on the kingdom of that monarch. Then, O chastiser of enemies, when that season of drought broke out, the people of that kingdom, as also the trees and lower animals began to die fast. And during the continuance of that dreadful drought, not even a drop of dew fell from the skies and no corn grew. And the inhabitants in despair, and afflicted with the fear of hunger, left their homes and fled away in all directions. And the famished people of the capital and the country began to abandon their wives and children and grew reckless of one another. The people being afflicted with hunger, without a morsel of food and reduced to skeletons, the capital looked very much like the city of the king of the dead, full of only ghostly beings. On beholding the capital reduced to such a state, the illustrious and virtuous and best of Ṛṣis, Vasiṣṭha was resolved upon applying a remedy and brought back unto the city that tiger among kings, Saṃvaraṇa, along with his wife, after the latter had passed so long a period in solitude and seclusion. After the king had entered his capital, things became as before, for, when that tiger among kings came back to his own, the god of a thousand eyes, the slayer of Asuras, poured rain in abundance and caused corn to grow.”
“While Saṃvaraṇa the son of Ṛkṣa, O king, was ruling the earth, there was a very great loss of people, so we have heard. The kingdom was shattered by manifold destructions in this way: struck by death for starvation, by want of rain and diseases, and the troops of the enemies attacked the Bhāratas. And shaking the Earth, so to say, with a fourfold army (i.e. made of chariots, elephants, knights and infantrymen), the Pañcāla marched against him (Saṃvaraṇa), and, having quickly conquered the Earth, he defeated him in battle with ten Akṣauhiṇis (troops of tenths of thousands of soldiers).”
"Towards the end of Tretā and the beginning of Dvāpara, a frightful drought occurred, extending over twelve years, in consequence of what the gods had ordained. At that time which was the end of Tretā and the commencement of Dvāpara, when the period came for many creatures superannuated by age to lay down their lives, the thousand-eyed deity of heaven poured no rain. The planet Bṛhaspati began to move in a retrograde course, and Soma abandoning his own orbit, receded towards the south. Not even could a dew-drop be seen, what need then be said of clouds gathering together? The rivers all shrank into narrow streamlets. Everywhere lakes and wells and springs disappeared and lost their beauty in consequence of that order of things which the gods brought about. Water having become scarce, the places set up by charity for its distribution became desolate. The Brahmanas abstained from sacrifices and recitation of the Vedas. They no longer uttered Vashats and performed other propitiatory rites. Agriculture and keep of cattle were given up. Markets and shops were abandoned. Stakes for tethering sacrificial animals disappeared. People no longer collected diverse kinds of articles for sacrifices. All festivals and amusements perished. Everywhere heaps of bones were visible and every place resounded with the shrill cries and yells of fierce creatures. The cities and towns of the earth became empty of inhabitants. Villages and hamlets were burnt down. Some afflicted by robbers, some by weapons, and some by bad kings, and in fear of one another, began to fly away. Temples and places of worship became desolate. They that were aged were forcibly turned out of their houses. Kine and goats and sheep and buffaloes fought (for food) and perished in large numbers. The Brahmanas began to die on all sides. Protection was at an end. Herbs and plants were dried up. The earth became shorn of all her beauty and exceedingly awful like the trees in a crematorium. In that period of terror, when righteousness was nowhere, O Yudhishthira, men in hunger lost their senses and began to eat one another. The very Ṛṣis, giving up their vows and abandoning their fires and deities, and deserting their retreats in woods, began to wander hither and thither (in search of food). The holy and great Ṛṣi Viśvāmitra, possessed of great intelligence, wandered homeless and afflicted with hunger."
"a drought, O king, occurred that extended for twelve years. During that drought extending for twelve years, the great rishis, for the sake of sustenance, fled away, O monarch, on all sides. Beholding them scattered in all directions, the sage Sārasvata also set his heart on flight. The river Sarasvatī then said unto him, 'Thou needst not, O son, depart hence, for I will always supply thee with food even here by giving thee large fishes! Stay thou, therefore, even here!' Thus addressed (by the river), the sage continued to live there and offer oblations of food unto the Ṛṣis and the gods. He got also his daily food and thus continued to support both himself and the gods. After that twelve year's drought had passed away, the great Ṛṣis solicited one another for lectures on the Vedas. While wandering with famished stomachs, the Ṛṣis had lost the knowledge of the Vedas. There was, indeed, not one amongst them that could understand the scriptures. It chanced that someone amongst them encountered Sārasvata, that foremost of Ṛṣis, while the latter was reading the Vedas with concentrated attention.”
“Purukutsa had a son by Narmada named Trasadasyu, whose son was Sambhūta, whose son was Anaraṇya, who was slain, by Rāvaṇa in his triumphant progress through the nations. The son of Anaraṇya was Pṛṣadaśva; his son was Haryyaśva; his son was Sumanas; his son was Tridhanvan; his son was Trayyāruṇa; and his son was Satyavrata, who obtained the appellation of Triśaṅku, and was degraded to the condition of a Caṇḍāla, or outcast. During a twelve years' famine Triśaṅku provided the flesh of deer for the nourishment of the wife and children of Viśvāmitra, suspending it upon a spreading fig-tree on the borders of the Ganges, that he might not subject them to the indignity of receiving presents from an outcast. On this account Viśvāmitra, being highly pleased with him, elevated him in his living body to heaven.”
In this account, we have the mention of Rāvaṇa, the terrible king of Lanka, who in his raid in the subcontinent (or in Lanka itself) was vanquished by the Haihaya king Arjuna Kārtavīrya, then imprisoned and later released, and, as is well known, he was killed by Rāma Dāśarathi. As also Lloyd remarks, here we have a clear synchronism of Arjuna and Rāma, and so we have to place also Arjuna in the period of the transition between Tretā and Dvāpara; also his act of burning the earth with its settlements and the following invasion, together with Haihayas, of Western raiders (identified as Śakas, Yavanas, Kāmbojas, Pāradas and Pahlavas in Vāyu Puraṇa 26.121-128, invading the kingdom of Bāhu, 8 generations after Triśaṅku) can be connected to the same period. This harmonizes with the date (2000 BCE) given to the beginning of the so-called "Malwa Culture" (see here, p.227), which was also in Maheshwar (identified with the royal town of Arjuna, Māhiṣmatī), and with the presence of objects similar to Iranian ones in Malwa sites, like spouted pots (see here). Iranian or Central Asian affinities were found also in the Cemetery H culture starting from 1900 BCE. These western peoples can also be those mentioned in RV VII.18 like the Pakthas. So, this period of climatic crisis brought various invasions, especially from Central Asia, but also internal movements like that of the Haihayas from Malwa and of Rāvaṇa from the South. A drought is also connected with Lomapāda, the king of Aṅga in Eastern India, friend of Daśaratha, the father of Rāma (see MBh. III.310), but without mention of 12 years, that we find instead in the story of another figure contemporary of Rāma according to Lloyd, Māndhātṛ.
5) Māndhātṛ. MBh III.126:
“When there was a drought, which continued for twelve consecutive years, the mighty king caused rain to come down for the growth of crops, paying no heed to Indra, the wielder of the thunder-bolt, who remained staring (at him). The mighty ruler of the Gandhara land, born in the lunar dynasty of kings, who was terrible like a a roaring cloud, was slain by him, who wounded him sorely with his shafts. O king! he of cultured soul protected the four orders of people, and by him of mighty force the worlds were kept from harm, by virtue of his austere and righteous life. This is the spot where he, lustrous like the sun, sacrificed to the god. Look at it! here it is, in the midst of the field of the Kurus, situated in a tract, the holiest of all. O preceptor of earth! requested by thee, I have thus narrated to thee the great life of Mandhata, and also the way in which he was born, which was a birth of an extraordinary kind.”It is significant that a Mandhātṛ is also cited in Ṛgveda (I.112, VIII.39, VIII.40), and he (called Māndhātṛ Yauvanāśva) is the poet of X.134 according to the Anukramaṇī, a hymn very similar to X.133 (sharing the meter mahāpaṅkti, phraseology and dedication to Indra), that is ascribed to Sudās and has also a refrain similar to RV VIII.39-40 and partially the same śakvarī and mahāpaṅkti meter. X.133 speaks of war, and the śakvarī meter is mentioned in the hymn of the battle of the Ten Kings, VII.33.4, as the meter of the cry that attracted Indra to fight on the side of Sudās (see the translation of Jamison and Brereton). So, we can suppose that they belong to the same age of Sudās, the king of the great battle, who is also a contemporary of Māndhātṛ in Lloyd's table.
We will conclude with the story of Agastya:
6) Agastya. MBh XIV.95:
"In olden days, O king, Agastya of great energy, devoted to the good of all creatures, entered into a Dīkṣā extending for twelve years. […] As Agastya, however, was engaged in that sacrifice of his, the thousand-eyed Indra, O best of the Bhāratas, ceased to pour rain (on the Earth). At the intervals, O king, of the sacrificial rites, this talk occurred among those Ṛṣis of cleansed souls about the high-souled Agastya, viz., 'This Agastya, engaged in sacrifice, is making gifts of food with heart purged of pride and vanity. The deity of the clouds, however, has ceased to pour rain. How, indeed, will food grow? This sacrifice of the Ṛṣi, ye Brahmanas, is great and extends for twelve years. The deity will not pour rain for these twelve years."
"Using a new high-resolution (~5 years/sample) speleothem stable isotope record from northeast India that spans the early and mid-Holocene, a number of abrupt changes in the oxygen isotopic composition of precipitation (δ 18 O p) are documented. The most dramatic of these events occurred ~4000 years ago when, over the course of approximately a decade, isotopic values abruptly rose above any seen during the early to mid-Holocene and remained at this anomalous state for almost two centuries. This event occurs nearly synchronously with climatic changes documented in a number of proxy records across North Africa, the Middle East, the Tibetan Plateau, southern Europe, and North America. We hypothesize that the excursion could represent a shift toward an earlier Indian Summer Monsoon withdrawal or a general decline in the total amount of monsoon precipitation. [...] the tight age constraints of the record show with a high degree of certainty that much of the documented deurbanization of the Indus Valley at 3.9 kyr B.P. occurred after multiple decades of a shift in the monsoon’s character but before the monsoon returned to its previous mid-Holocene state." "The most isotopically enriched values of the entire record occur between 4071 B.P. (±18 years) and 3888 B.P. (±22 years) during which the calcite remained enriched by ~0.8‰ relative to modern values (1.5‰ relative to the background values of the time) for a period of 183 years. The isotopic changes at this time manifested as a two-step process where values experienced a small steplike rise between ~4315 and 4303 years B.P. and experienced a second and more precipitous rise between ~4071 and 4049 years B.P. The abrupt shift occurred over approximately two decades, after which the values stabilized at this relatively enriched state for ~180 years before rapidly returning to previous background values at 3888 years B.P." "The monsoon over northeast India appears to have experienced an abrupt excursion at 4000 years B.P., the magnitude of which, in terms of both amplitude and length, exceeds any other event during either the most recent 600 years or throughout the early to mid-Holocene."
In this diagram from the paper (p.77) we see how around 4000 years BP we have a sudden fall of precipitation. In a supplementary list of dates the date of the sharp diminution of rain is 4055 BP, a sharp increase in 3941 BP, followed by another decrease, and finally a stable increase from 3899 BP. The lowest point is between 4049 and 4037 BP. Taken exactly, since BP is based on 1950 CE, this suggests a special drought in South Asia between 2099 and 2087 BCE. The error given is 30 years. Also a paper by Dixit of 2014 ("Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago"), about the Kotla Dahar lake in Haryana, gives a similar date: "An abrupt 4‰ increase in δ18Oa occurred at ca. 4.1 ka, documenting a sharp reduction in Indian Summer Monsoon intensity."