Climate and Global Change Issues: Troposphere Temperatures
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The estimated global mean temperature anomaly (land only) for 1994, calculated relative to the 1951-1980 base period, increased to +0.4C. This value is comparable to that observed during 1991, but smaller than the record 1990 anomaly. Thus, 1994 is one of the three warmest years on record (Fig. 1.1, top). The global temperature anomalies were estimated from station data received over the Global Telecommunications System (GTS). Most of the warmth during 1994 occurred after February, as global temperatures during the March - December period were the warmest observed during the historical record dating back to 1951 (Fig. 1.1, bottom). In addition, individual monthly records for mean global temperatures were set during April, September, and December.
Median global temperatures (land only) were above normal during every month of 1994 except February (Fig. 1.2). In addition, 70% or more of the land area (with data) had above-normal temperatures during April, June, and December. It should be noted, however, that at least 30% of the total land area experienced below-normal temperatures during every month, except for December. This is similar to time series for both 1990 and 1991 (see Climate Assessment 1991, Fig. 1.2), although the median anomalies during those years were warmer than during 1994.
The 1994 estimated global mean surface temperature for land and marine areas was +0.31C, calculated relative to the 1951-1980 base period (Figure 1.3). The uncertainty in estimates of global temperatures results mainly from data sparsity, especially over the Southern Ocean, parts of the tropics, and Antarctica. As a result, different analysis techniques yield slightly different estimates of the global mean temperature anomaly. All estimates for 1994 rely heavily on land surface air temperature data from the monthly CLIMAT messages and on oceanic surface temperatures obtained from ship and buoy measurements. These data are exchanged over the GTS.
Figure 1.3 shows that 1994 was warmer than 1992 and 1993, was similar to 1988, but was not as warm as 1990 and 1991. The time series also shows that global temperatures have increased approximately 0.5C over the past 135 years. Much of this increase has occurred during two periods, a thirty-year period from approximately 1910 to 1940, and over the past 15 years.
The abnormally warm conditions during 1994 partly resulted from the redevelopment of El Nino conditions during the second half of the year (see section 2). The El Nino influence is evident in the geographical patterns of ocean temperature anomalies (Figure 1.4), which shows warmth in the central equatorial Pacific and along the west coast of North America, warmth in the central Indian Ocean, and cold areas centered north of Hawaii and in the western subtropical South Pacific. Positive land temperature anomalies, exceeding 1C, were observed over western North America, central and eastern Europe (where the summer was exceptionally hot, see section 4) and eastern Asia. Abnormally warm conditions have been observed in northwestern Canada during recent years. The anomalous warmth over Asia is farther south than the persistent pattern that has dominated Siberia during the past 15 years. In contrast, Labrador, southern Greenland, and the northwestern North Atlantic were anomalously cold during 1994, as was observed during 1993 and in many other recent years. The overall distribution of temperature anomalies during 1994 was similar to that observed during the record warm year of 1990.
Volcanic aerosols from the June 1991 eruption of Mt. Pinatubo have decreased to near pre- eruption levels, as shown by the "apparent" atmospheric solar transmission, or transmission ratio (Ellis and Pueschel 1971) (Fig. 1.5). Consequently, the global cooling influence from the aerosols waned (Fig. 1.6), as indicated by surface observations and model simulations from the Goddard Institute for Space Studies climate model (Hansen et al. 1988, 1992). The model is designed to simulate the global temperature change due to atmospheric aerosols and trace gases and does not incorporate the global temperature response associated with processes such as ENSO. The simulations and observations agree that a maximum surface cooling of approximately 0.5C occurred by the end of 1992 in association with the eruption of Mt. Pinatubo. The model forecasts suggest that during 1994 global temperatures should have returned to pre-eruption levels. Due to large increases late in the year, observed temperatures had also returned to pre-eruption values by the end of 1994 (Fig. 1.6).
Climate and Global Change Issues: Troposphere Temperatures
Executive Summary
Table of Contents
