How Hot was the Oligocene? When studying the paleoenvironment of the Oligocene epoch, a period in Earth's history spanning from 34 to 23 million years ago, I had a really hard time understanding what the climate was like in comparison to today:


  The Paleogene period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2°C, 400,000-year temperature depression leads the 2°C, seven-million-year stepwise climate change 33.5 Ma (million years ago). The stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55 m (181 feet) decrease in sea level (35.7–33.5 Ma) with a closely related (25.5–32.5 Ma) temperature depression. The 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma. A deep 400,000-year glaciated Oligocene–Miocene boundary event is recorded at McMurdo Sound and King George Island.


This statement from Wikipedia struck me as vague and therefore confusing. So in plain, simple English and in general, what was the climate of the Oligocene like in comparison with today's climate?
Modified from Cramer et al. 2009 by adjusting to the Gradstein et al. 2012 timescale. In black, it is the Pacific trend; in blue the North Atlantic and in green the South Atlantic.

This graph shows the Cenozoic variations of $\delta\ce{^{18}O}_{Cibicidoides}$ according to Cramer et al. 2009. This proxy is thought to vary according to variations of deep sea temperature (to simplify) and thus should reflect the climate state at a given time. As negative values of $\delta\ce{^{18}O}_{Cibicidoides}$ corresponds to warmer temperature than positive ones, the graph is oriented accordingly. I highlighted the beginning (33.9Ma) and end (23.03Ma) of the Oligocene in red.

The Oligocene starts with a rapid cooling (linked to the East Antarctic glaciation, see for instance Kennett 1977, Zachos et al. 2001, De Conto &amp; Pollard 2003). Yet as you can see on the graph the temperature is still warmer than the late Miocene to Recent period. Some regional trends then show a moderate warming at the end of the Oligocene that culminates in the middle Miocene climatic Optimum, ca. ten million years later.