Lesson 3 Summary

In this lesson, we reviewed key observations that detail how our atmosphere and climate are changing. We have seen that:

• Ice in its various forms (sea ice, glaciers, and ice sheets) is disappearing as the globe warms;
• Global sea level is rising due both to the expansion of warming seawater and the contribution from melting glaciers and ice sheets;
• The deep ocean, as well as the ocean surface, is warming in a manner consistent with a warming climate; the slow nature of the ocean warming means that global sea level will continue to rise for several centuries, even if we were to freeze greenhouse concentrations at current levels;
• Ocean circulation trends are difficult to establish, but there is some reason to believe that the ocean's thermohaline circulation may weaken;
• There is substantial natural multidecadal variability that appears related to oscillations in the strength of the ocean's thermohaline circulation;
• The El Niño/Southern Oscillation (ENSO) is the most prominent mode of climate variability on interannual timescales, and one important uncertainty in projecting future climate change involves uncertainties in how ENSO will change in the future;
• There is a trend toward increasingly powerful Atlantic Hurricanes over the past half century, and this increase appears to mirror warming tropical Atlantic sea surface temperatures;
• There is also a trend toward increased Atlantic tropical cyclone counts, though the reliability of the data, particularly in earlier decades, has been called into question. A variety of factors, including sea surface temperatures, El Niño, and the so-called North Atlantic Oscillation (NAO) atmospheric circulation pattern, influence Atlantic tropical cyclone counts;
• Other extreme weather events appear to have become more common in recent decades. The increased frequency and duration of heat waves around the world is likely related to large-scale warming; there is also some evidence that mid-latitude storm systems have become more powerful, consistent with predictions of climate models;
• Paleoclimate evidence demonstrates that CO₂ has been the 'major lever' influencing global climate change over geological time;
• CO₂ and CH₄ greenhouse gas forcing is a key component in the glacial/interglacial cycles of the late Pleistocene, with ice albedo feedback playing an especially important role, and with changes in Earth orbital geometry providing the pacing, including the dominant 100 kyr oscillations of the past 700,000 years;
• The Younger Dryas cooling, even in the North Atlantic, that occurred at the termination of the last ice age demonstrates the potentially important role of ocean dynamical responses to meltwater fluxes; such effects are unlikely to be as important in the context of modern climate change;
• Earth orbital changes have influenced climate changes over the course of the current interglacial period (the Holocene). Increased high-latitude summer insolation was responsible for the relatively warm summers at high Northern latitudes during a period from roughly 10,000-6,000 years ago known as the Holocene optimum. The same seasonal and latitudinal redistribution of solar insolation was responsible for changes in atmospheric circulation such as the strengthened west African Monsoon that led to the greening of the Sahara at this time;
• Temperatures over the past millennium have been dominated by other radiative forcings, such as natural forcing by volcanic eruptions and solar insolation changes, and anthropogenic forcing over the past two centuries;
• The warming of the past few decades appears to exceed the range seen for at least the past millennium.

Reminder - Complete all of the lesson tasks!

You have finished Lesson 3. Double-check the list of requirements on the first page of this lesson to make sure you have completed all of the activities listed there before beginning the next lesson.