원문언어
여름의 더위와 겨울의 추위는 지구의 자전축이 기울어진데서 비롯한다. 이것은 특정 지역에서 한 해를 통해 얼마나 많은 양의 태양빛이 지구에 도달하느냐를 결정한다.
몇몇 과학자들은 과거엔 이러한 기울어짐이 훨씬 심했었고 이것으로 적도 지방이 추웠던 시기를 설명할 수 있다고 생각하고 있다. 현재 연구자들은 지구가 어떻게 스스로 똑바로 설 수 있었는지에 대해 설명을 할 수 있다. Nature지 다음 호에 발표될 그들의 각본에 의하면 이것은 지구의 모양을 바꾸어 놓는 거대한 얼음층들의 증가와 감소에 달려 있다.
오늘날 지구의 자전축은 지구의 공전 궤도면에 23.5도 만큼 기울어져 있다. 이러한 자전축의 경사는 여름에는 극지방에 적도지방보다 더 적은 양의 햇빛이 도달하게 하고 겨울에는 전혀 도달하지 못하게 만든다. 그러나 지질학자들은 뒤섞인 암석들 속에 빙하가 열대 지방을 덮었던 흔적이 남아있는 6억 년 전을 생각해 보았다.
이것은 전체 행성이 빙하로 뒤덮였다는 것을 의미하는 것일까? 많은 연구자들은 이러한 "눈덩이의 지구"에 반대한다.
왜냐하면 그러한 조건하에서 생명체가 어떻게 살아남을 수 있었을까 하는 의문 때문이다. 대신에 그들은 지구의 자전축이 54도 이상으로 기울어졌었을 것이라고 주장한다. 그것은 많이 누워서 도는 팽이와도 비슷하다. 그러면 적도 지방은 모든 것이 얼어붙은 반면 극지방이 온난한 시기에 들어갔을 것이다.
그러나 문제는 어떻게 지구가 오늘날의 경사각도를 갖게
되었느냐 하는 것이다. 달, 태양, 그리고 다른 행성들에 의해서 생겨나는 조수의 힘은 지구의 모양에 의존하기 때문에 지구 물리학자들은 지구의 모양을 좀 더 둥글게 만들거나 덜 둥글게 만들 수 있는 빙하의 양에 대한 효과를 모형화했다. 만일 큰 얼음 덩어리가 적도로부터 극지방으로 흘러가면 지구는 그 모양이 서서히 변화된다. 연구자들은 이러한 변화가 지구 자전에 영향을 줘서 1억 년 이내에 수십 도정도의 기울어짐을 바로 잡을 수 있다는 것을 발견했다.
"기후와 빙하와 경사 각도 사이의 상호 작용은 매우
복잡합니다. 우리는 특정한 환경 조건 안에서 그것이 효력을 갖게 만들 수 있다는 것을 보여주었습니다."라고 논문의 공저자 James Kasting은 말했다. 그는 University Park에 있는 펜실베니아 주립 대학교에서 일하고 있다.
메릴랜드 그린벨에 있는 나사의 고다드 우주비행센터
지구물리학자 Bruce Bills는 이 이론이 매우 그럴 듯하다고 말한다.
"그러나 모든 사람이 경사각이 그렇게 컸다는 것을 확신하지는 않습니다. 그래서 이것은 존재하지 않았던 현상에 대한 하나의 설명이 될 수도 있습니다."라고 그는 덧붙였다. Bills와 Kasting은 둘 다 고대 지구에 있었던 빙하의 시기와 지리적인 패턴을 측정하기 위해 빙하 퇴적물에 대한 더 많은 연구를 계획하고 있다.
원문출판날짜 1998년 12월 02일
국가코드 미국
주제분야 지구물리
Copyright(c) 1998, KORDIC All right reserved Email comments
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An oblique view of climate
BRUCE G. BILLS
Nature 396, 405 - 406 (1998) ⓒ Macmillan Publishers Ltd.
3 December 1998
One explanation for certain patterns of glaciation in the
past invokes a large and comparatively swift decline in the
tilt, or obliquity, of the Earth. A provocative hypothesis
provides a mechanism by which such a decline could have
occurred.
How do variations in Earth's orbital and rotational
geometry influence climate? Does the climate system, in turn,
influence rotation? We all experience the radiative and thermal
cycles of night and day, winter and summer. So we are familiar
enough with the influence of Earth's rotational and orbital
motions on the spatio-temporal pattern of light and temperature
to make it easy to imagine how long-term variations in the
orbit and rotation would affect climate. Much recent data and
modelling help confirm that principle1,2.
Somewhat further removed from human experience is the
notion that climatic change itself could influence the
rotational dynamics of the Earth. That, however, is the
hypothesis advanced by Williams et al. on page 453 of this
issue3. The basic idea is quite simple, and involves feedback
between Earth's obliquity (the angular separation between the
spin pole and orbit pole around the Sun, Fig. 1) and its
oblateness (departure from spherical symmetry).
Figure 1 Earth's obliquity. Full legend
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Figure 1 Earth's obliquity. Williams and colleagues' work3
stems from the need to account for a 20-30° decline in
obliquity, required by a hypothesis that explains the
occurrence of low-latitude glacial deposits 750-550 Myr ago
through an obliquity of more than 54° (a), when the equatorial
regions would have been colder than the poles. b, Since about
430 Myr, obliquity is thought to have been around 23.5°. The
authors propose that ice-sheet formation and configuration (not
shown) was the driving force.
First, however, it is useful to recall how orbital and
rotational geometry influences climate. The main seasonal cycle
is primarily determined by the orientation of the spin axis,
and only secondarily by the eccentricity of the orbit.
Currently, perihelion (Earth's closest approach to the Sun)
occurs several weeks after winter solstice in the Northern
Hemisphere (shortest daylight). However, neither the orbit nor
the spin axis is fixed in space. Gravitational interaction with
other planets (principally Venus) causes the shape and
orientation of the orbit to change on a variety of timescales4,
with the dominant period near 70,000 years (70 kyr) and
subsidiary oscillations at periods ranging from 50 kyr to 1.9
million years (Myr).
Gravitational torques exerted by the Moon and Sun on the
oblate figure of the Earth cause the spin axis to precess with
a period of 25.8 kyr. If the orbit plane were fixed, the path
of the spin pole would be a circle centred on the orbit pole,
keeping the obliquity fixed. However, because the orbit is also
precessing, the obliquity oscillates by 1° about its present
value of 23.5°, with a period of 41 kyr. These obliquity
oscillations modulate the seasonal and latitudinal pattern of
incident radiation, and thus affect climatic variations.
How then does climate change influence rotation? One way is
to change the spin precession rate by changing the oblateness
of the Earth's mass distribution. During major glacial cycles,
mass transport between the oceans and ice sheets is sufficient
to change the precession rate by about 1%. The net change
includes accumulation of continental ice and partially
compensating subsidence of the Earth's surface. If the
obliquity and oblateness oscillations are exactly in phase,
there is no long-term net effect. But if the oblateness lags
behind the obliquity, there will be a secular change in
obliquity, with a rate that depends on the amplitude and phase
of the oblateness variations5-7.
The long-term stability of Earth's climate system is an
important question, but one that remains elusive. Despite
progress in short-term weather prediction (based on improved
quality and quantity of observations, faster computers and
better understanding of the system dynamics), our understanding
of long-term climate dynamics is still quite primitive. Part of
the problem, of course, is that the further back into the past
we go, the more difficult it is to reconstruct which path the
climate system has followed. When we still don't know what has
happened, how can we reconstruct why? In this situation, the
role of theoretical models such as that of Williams et al.3 is
not so much to explain what actually happened as to broaden our
perspectives on the types of behaviours that might have
oc
curred.
The principal novelty of the new work by Williams et al.
concerns the rate and direction of the obliquity change they
seek to explain (Fig. 1). They are invoking an
obliquity-oblateness feedback to decrease Earth's obliquity,
rather than increase it, and the obliquity change they seek is
20-30° in less than 100 Myr. They propose that such a change
could have been caused by the very large ice sheets that might
have built up during periods when the continents were clustered
near to the poles.The motivation for that particular scheme
comes from a suggestion that Earth's obliquity has been large
throughout much of its history8 (in excess of 54°, the value
at which the annual average incident radiation at the Equator
equals that at the poles; at greater obliquities, the
equatorial regions b
ecome colder than the poles). That
suggestion was made in an attempt to explain evidence for
low-latitude glacial deposits from the Neoproterozoic (750-550
Myr ago), while avoiding a 'snowball Earth', in which the
entire surface of the Earth would be covered by ice9-12.
As always, more work is needed. In this case,
distinguishing between the two competing climatic possibilities
(equatorial versus global glaciation) should be easily
resolvable by searching for contemporaneous high-latitude and
low-latitude glacial deposits13. Reconstructing an unambiguous
obliquity history will be more of a challenge.
Bruce G. Bills, currently at the Institute of Geophysics
and Planetary Physics, Scripps Institution of Oceanography, is
in the Geodynamics Branch, NASA Goddard Space Flight Center,
Mailstop 921.0, Greenbelt, Maryland 20771, USA.
e-mail: bills@denali.gsfc.nasa.gov