MPSC 2024 Prelims Paper-I · Question 31 of 100

Correct — option (4), All of the above. Each of the three items named is a recognised palaeoclimate proxy, that is, a natural archive that recorded the climate of its own time and can be read long afterwards, and together they cover three different timescales, which is exactly why the combination is stronger evidence than any one of them alone. Tree rings, option (1), give annual resolution: a tree lays down one growth ring a year, and the width and density of that ring vary with the conditions of the growing season. At high altitude and near the treeline, growth is limited chiefly by temperature rather than by anything else, so rings from such sites are unusually clean temperature recorders, and overlapping living trees with dead and buried wood extends the record back centuries or millennia. Glacial advances and retreats, option (2), give the long view: moraines, erratic boulders, striated rock surfaces and over-deepened valleys mark where ice once stood, and mapping them shows that ice sheets and mountain glaciers have expanded and contracted repeatedly through the Quaternary, in glacial and interglacial cycles that ran long before any human influence on the atmosphere. Sediment deposits in glacial lakes, option (3), fill the gap between the two: meltwater feeding a glacial lake deposits a coarse layer in the summer melt season and a fine layer in winter when the lake is still or frozen, so the sediment accumulates in annual couplets, called varves, which can be counted like tree rings to date the layers and read for changes in the amount of melt. Since all three are valid lines of evidence and each independently shows climate varying long before industrial times, no single-item option can be complete and option (4) is the answer.

• (1)Tree rings in high altitudes — Tree rings are genuine and excellent evidence, so this option is not false in what it asserts — it is wrong only in what it excludes. Dendroclimatology gives an annually resolved record, but it reaches back at most a few thousand years even with buried and sub-fossil wood, and it is limited to places where suitable trees grow. That range says nothing about the glacial cycles of the Quaternary, which are the strongest demonstration that climate has always changed. Choosing this option alone means treating one proxy with a short reach as the whole of the evidence.
• (2)Glacial advances and retreats — This too is sound evidence taken on its own — moraines, erratics and glacial striations record ice sheets expanding and contracting repeatedly through the Quaternary, entirely without human involvement. But the glacial record is coarse in time: it shows that major cold and warm phases alternated, not what happened decade by decade within them. Excluding tree rings and lake varves removes precisely the fine-resolution evidence that shows climate varies continuously rather than only in great swings, and the stem asks about a natural and continuous process.
• (3)Sediment deposits in glacial lakes — Varved sediments in glacial lakes are a real and valuable archive, laid down in annual couplets of a coarse summer layer and a fine winter layer that can be counted to build a dated chronology of melt and cold. As a lone answer, however, it is the narrowest of the three: it depends on the survival of an undisturbed lake basin, and it is available only where glacial meltwater fed a standing body of water. Rejecting the tree-ring and glacial-landform evidence in its favour would discard both the annually resolved record with the widest geographical coverage and the record with the longest reach in time.

Instrumental weather records cover only the last century and a half, so everything known about climate before that comes from proxies — natural materials whose properties were set by the climate of the time and can be measured now. The main families are biological, geological and chemical. Tree rings are the classic biological proxy, giving annual resolution over centuries to millennia; pollen preserved in bog and lake sediment shows what vegetation grew and therefore what climate prevailed. Geological proxies include glacial landforms — moraines, erratics, striations, drumlins — which map the former extent of ice, and varved lake sediments, which record annual melt. Chemical proxies include the oxygen isotope ratios locked in ice cores from Greenland and Antarctica and in the shells of marine microfossils in ocean sediment, which reach back hundreds of thousands to millions of years. Read together these archives show a climate that has never been steady: the Quaternary alone contains repeated glacial and interglacial cycles, driven in large part by slow, regular changes in the earth's orbit and axial tilt, with volcanic eruptions and variations in solar output adding shorter-term fluctuations. Establishing that natural variability is also what allows scientists to judge how far a later change departs from it.

Environment and climate questions in MPSC papers are often framed around the idea that climate change is not new, and the expected answer is a list of the evidence for past change rather than an argument about the present. This question is built as three genuine proxies plus an 'all of the above' option, which is the Commission's usual way of testing whether a candidate knows the range of evidence or only the one example that appears most often in textbooks. The habit it rewards is to test each option on its own merits before considering the composite option: if none of the three can be shown to be invalid evidence, the composite must be the answer. It also rewards knowing why several proxies are used together — because they differ in resolution and in reach, and a claim about climate is only as strong as the number of independent archives that agree on it.

• A palaeoclimate proxy is a natural archive whose properties were set by the climate of its time — tree rings, glacial landforms, lake varves, pollen, ice cores and ocean sediments are the main types.
• Tree rings give annual resolution, and rings from high-altitude and treeline sites are especially useful because growth there is limited chiefly by temperature.
• Moraines, erratic boulders and striated rock surfaces map the former extent of ice and show that glaciers and ice sheets advanced and retreated repeatedly through the Quaternary.
• Varves are annual couplets of sediment in a glacial lake — a coarse summer layer from the melt season and a fine winter layer — which can be counted to date the record.
• Slow, regular changes in the earth's orbital eccentricity, axial tilt and precession are a principal driver of the glacial-interglacial cycles recorded by these archives, with volcanic eruptions and solar variation contributing shorter-term change.

• Rejecting an 'all of the above' option on principle; here each of the three items is independently valid evidence and no ground exists for excluding any of them
• Assuming tree rings are a biological rather than a climatic record and dismissing them as evidence about climate at all
• Confusing varves, which are annual sediment couplets in a glacial lake, with ordinary stratified sediment that carries no annual signal
• Treating evidence of past natural change as though it settled a question about the causes of recent change; the archives establish the natural variability against which any later change is measured
• The stem says "geological evidence", but tree rings are a BIOLOGICAL proxy (dendroclimatology) and ice cores are usually classed as glaciological or chemical — only the glacial advances and retreats are geological in the strict sense. The commission's own wording is loose here, in both the English and the मराठी column, and the key is still "all of the above": the question is testing whether you recognise the three archives, not whether you can sort them by discipline. Do not talk yourself out of an option because it sits outside the stem's adjective — but do notice that the adjective is wrong, because an examiner who used it precisely would have expected a different answer.

MPSC's environment section asks climate change from two directions: the evidence for past change, as here, and the mechanisms and policy of present change — greenhouse gases, the UNFCCC framework, the conferences of the parties and India's commitments. Questions on the evidence are usually list-based, with three or four genuine proxies offered and a composite option, and they reward breadth over depth. Questions on mechanisms are usually single-fact, naming a gas, a protocol or an institution. Both types recur, so it is worth holding the proxy list and the treaty list as separate blocks.

No directly related past PYQ was found.

• practice — not a real PYQ

  Varves, used in the reconstruction of past climate, are best described as which of the following ?

  • (a)Annual layers of sediment deposited in a glacial lake, one coarse and one fine per year
  • (b)Growth rings formed annually in the trunks of trees
  • (c)Air bubbles trapped in glacier ice preserving ancient atmosphere
  • (d)Bands of volcanic ash used to correlate sediment sequences

  Answer(a) Annual layers of sediment deposited in a glacial lake, one coarse and one fine per year — the coarse layer is laid down during the summer melt season and the fine layer when the lake is still or frozen in winter, so each couplet represents one year and the sequence can be counted. Tree rings, ice-core bubbles and ash bands are all separate archives.

• practice — not a real PYQ

  Tree rings taken from high-altitude and treeline sites are considered especially valuable climate proxies chiefly because of which of the following ?

  • (a)Trees there grow faster and produce wider rings
  • (b)Growth there is limited chiefly by temperature, so ring width records it clearly
  • (c)High-altitude trees live longer than any other species on earth
  • (d)Rings at high altitude form twice a year, doubling the resolution

  Answer(b) Growth there is limited chiefly by temperature, so ring width records it clearly — at lower sites moisture, soil and competition confuse the signal, whereas near the treeline temperature is the binding constraint and the rings become a comparatively clean thermometer. A tree lays down one ring a year, not two.