El Niño/Southern Oscillation (ENSO) is the primary mode of interannual climate variability, and understanding its response to climate change is critical, but research remains divided on the direction and magnitude of that response. Some twenty-first-century simulations suggest that increased CO₂ strengthens ENSO, but studies suggest that on palaeoclimate timescales higher temperatures are associated with a reduced ENSO amplitude and a weaker Pacific zonal temperature gradient, sometimes termed a ‘permanent El Niño’. Internal variability complicates this debate by masking the response of ENSO to forcing in centennial-length projections. Here we exploit millennial-length climate model simulations to disentangle forced changes to ENSO under transient and equilibrated conditions. On transient timescales, models show a wide spread in ENSO responses but, on millennial timescales, nearly all of them show decreased ENSO amplitude and a weakened Pacific zonal temperature gradient. Our results reconcile differences among twenty-first-century simulations and suggest that CO₂ forcing dampens ENSO over the long term.

厄尔尼诺/南方涛动 (ENSO) 是年际气候变率的主要模式，了解其对气候变化的响应至关重要，但对该响应的方向和幅度的研究仍然存在分歧。一些 21 世纪的模拟表明，增加的 CO ₂加强ENSO，但研究表明，在古气候时间尺度上，较高的温度与降低的ENSO幅度和较弱的太平洋纬向温度梯度有关，有时被称为“永久性厄尔尼诺”。内部可变性掩盖了 ENSO 对强迫进行百年长度预测的反应，从而使这场辩论变得复杂。在这里，我们利用千年长度的气候模型模拟来解开在瞬态和平衡条件下对 ENSO 的强制变化。在瞬态时间尺度上，模型显示 ENSO 响应分布广泛，但在千年时间尺度上，几乎所有模型都显示 ENSO 幅度减小和太平洋纬向温度梯度减弱。我们的结果调和了 21 世纪模拟之间的差异，并表明 CO ₂ 从长远来看，强迫会抑制 ENSO。