Climate has changed on all time scales throughout Earth’s history. Some aspects of the current
climate change are not unusual, but others are. The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years,
and has done so at an exceptionally fast rate. Current global temperatures are warmer than they
have ever been during at least the past five centuries, probably even for more than a millennium.
If warming continues unabated, the resulting climate change within this century would be extremely
unusual in geological terms. Another unusual aspect of recent climate change is its cause: past
climate changes were natural in origin (see FAQ 6.1), whereas most
of the warming of the past 50 years is attributable to human activities.
When comparing the current climate change to earlier, natural ones, three distinctions must be made.
First, it must be clear which variable is being compared: is it greenhouse gas concentration or
temperature (or some other climate parameter), and is it their absolute value or their rate of change?
Second, local changes must not be confused with global changes. Local climate changes are often much
larger than global ones, since local factors (e.g., changes in oceanic or atmospheric circulation)
can shift the delivery of heat or moisture from one place to another and local feedbacks operate
(e.g., sea ice feedback). Large changes in global mean temperature, in contrast, require some global
forcing (such as a change in greenhouse gas concentration or solar activity). Third, it is necessary
to distinguish between time scales. Climate changes over millions of years can be much larger and
have different causes (e.g., continental drift) compared to climate changes on a centennial time scale.
The main reason for the current concern about climate change is the rise in atmospheric carbon dioxide
(CO2) concentration (and some other greenhouse gases), which is very
unusual for the Quaternary (about the last two million years). The concentration of
CO2 is now known accurately for the past 650,000 years from antarctic
ice cores. During this time, CO2 concentration varied between a low of
180 ppm during cold glacial times and a high of 300 ppm during warm interglacials. Over the past
century, it rapidly increased well out of this range, and is now 379 ppm (see Chapter 2). For comparison,
the approximately 80-ppm rise in CO2 concentration at the end of the
past ice ages generally took over 5,000 years. Higher values than at present have only occurred many
millions of years ago (see FAQ 6.1).
Temperature is a more difficult variable to reconstruct than CO2 (a
globally well-mixed gas), as it does not have the same value all over the globe, so that a single
record (e.g., an ice core) is only of limited value. Local temperature fluctuations, even those over
just a few decades, can be several degrees celsius, which is larger than the global warming signal
of the past century of about 0.7°C.
More meaningful for global changes is an analysis of large-scale (global or hemispheric) averages,
where much of the
local variation averages out and variability is smaller. Sufficient coverage of instrumental records
goes back only about 150 years. Further back in time, compilations of proxy data from tree rings,
ice cores, etc., go back more than a thousand years with decreasing spatial coverage for earlier
periods (see Section 6.5). While there are differences among those reconstructions and significant
uncertainties remain, all published reconstructions find that temperatures were warm during medieval
times, cooled to low values in the 17th, 18th and 19th centuries, and warmed rapidly after that. The
medieval level of warmth is uncertain, but may have been reached again in the mid-20th century, only
to have likely been exceeded since then. These conclusions are supported by climate modelling as well.
Before 2,000 years ago, temperature variations have not been systematically compiled into large-scale
averages, but they do not provide evidence for warmer-than-present global annual mean temperatures
going back through the Holocene (the last 11,600 years; see Section 6.4). There are strong indications
that a warmer climate, with greatly reduced global ice cover and higher sea level, prevailed until
around 3 million years ago. Hence, current warmth appears unusual in the context of the past millennia,
but not unusual on longer time scales for which changes in tectonic activity (which can drive natural,
slow variations in greenhouse gas concentration) become relevant (see Box 6.1).
A different matter is the current rate of warming. Are more rapid global climate changes recorded
in proxy data? The largest temperature changes of the past million years are the glacial cycles,
during which the global mean temperature changed by 4°C to 7°C between ice ages and warm
interglacial periods (local changes were much larger, for example near the continental ice sheets).
However, the data indicate that the global warming at the end of an ice age was a gradual process
taking about 5,000 years (see Section 6.3). It is thus clear that the current rate of global climate
change is much more rapid and very unusual in the context of past changes. The much-discussed abrupt
climate shifts during glacial times (see Section 6.3) are not counter-examples, since they were probably
due to changes in ocean heat transport, which would be unlikely to affect the global mean temperature.
Further back in time, beyond ice core data, the time resolution of sediment cores and other archives
does not resolve changes as rapid as the present warming. Hence, although large climate changes have
occurred in the past, there is no evidence that these took place at a faster rate than present warming.
If projections of approximately 5°C warming in this century (the upper end of the range) are realised,
then the Earth will have experienced about the same amount of global mean warming as it did at the end of
the last ice age; there is no evidence that this rate of possible future global change was matched by any
comparable global temperature increase of the last 50 million years.